Ah, *less* vs. *fewer*. Another arrow in the prescriptivist’s quiver of pointless pedantry.

There's even a [Wikipedia article about the dispute][1]. There is also a [Language Log entry][2] about the matter too.

According to [Merriam-Webster's Dictionary of English Usage][3], a usage guide that looks carefully at the history of usage advice, the rule creating a clear separation for *less* and *fewer* was invented in 1770 by Rober Baker in his book _Reflections on the English Language_, where he wrote in [a comment on *less*][4]:

> The Word is most commonly used in speaking of a Number; where I should think *Fewer* would do better. *No Fewer than a Hundred* appears to me not only more elegant than *No less than a Hundred*, but more strictly proper.

The Merriam-Webster Dictionary of English Usage authors then comment:

> Baker’s remarks about *fewer* express clearly and modestly—“I should think,”, “appears to me”—his own taste and preference. It is instructive to compare Baker with one of the most recent college handbooks in our collection:
>>*Fewer* refers to quantities that can be counted individually.… *Less* is used for collective quantities that are not counted individually… and for abstract characteristics. —Trimmer & McCrimmon 1988

> Notice how Baker’s preference has here been generalized and elevated to an absolute status, and his notice of contrary usage has been omitted. This approach is quite common in handbooks and schoolbooks; many pedagogues seem reluctant to share the often complicated facts about English with their students.

> How Baker’s opinion came to be an inviolable rule, we do not know. But we do know that many people believe it is such.

They then give many examples of usage of less for countable quantities, and add finally:

>The examples above show native speakers and writers of English using *less* of count nouns in various constructions. *Fewer* could have been used in many of them—at times it might have been more elegant, as Robert Baker thought—but in others no native speaker would use anything but *less*.

With regards to the example in the original question, either *fewer* or *less* would be perfectly grammatical, but so many people are under the spell of the rule that *less* must never be used with countable nouns that anyone who doesn’t follow the rule may be subject to criticism.

**Edit 2010-09-28:**

The Cambridge Grammar of the English Language also weighs in on *less* vs. *fewer*:

>The relation between *less* and *fewer* is fairly complex. In non-count singulars only *less* is possible: *Kim has less/\*fewer money than Pat.* In plural NPs we have:  
> [17]  
> i. *She left less than ten minutes ago.*  
> ii. *Less/Fewer than thirty of the students had voted.*  
> iii. *He made no less/fewer than fifteen mistakes.*  
> iv. *You pass if you make ten mistakes or less/?fewer*.  
> v. *He took less/\*fewer pains to convince us than I’d expected*.  
> vi. *He made fewer/less mistakes than the others.*

> Both [i] and [ii] have *than* + numeral. In [i] *ten minutes* expresses an amount of time rather than a number of individuated units, and in such cases *fewer* is virtually impossible—just as *few* would be in a comparison of equality: *She left as little/\*few as ten minutes ago.* Similarly with *We paid less than thirty dollars for it; She’s less than forty years old; We were going at less than ten miles an hour.* In [ii] we are concerned with countable individuals and *little* cannot be used in a comparison of equality (*\*as little as thirty of the students*); nevertheless, for inequality *less* is more common than *fewer* in this construction. The same applies with percentages: *Less/Fewer than 30% of the students had voted.* Construction [iii] has the comparative form following *no*: though the interpretation is count plural, *less* is here again more common than *fewer*. Construction [iv] has *or* after a numeral: *less* is the usual form here, with *fewer* quite marginal; this construction is widely seen in supermarkets, with the fast checkout labelled *eight items or less*, or the like. In [v] *pains* is plural but non-count rather than count (we can’t ask how many pains he took), and here only *less* is possible. Finally in [vi] (as also in [15ii]) the comparative occurs directly with a count plural noun: both forms are found, but *less* is subject to quite strong prescriptavist disapproval, so that *fewer* is widely preferred in formal style, and many speakers in informal style too. 

> [Usage manuals are divided on the issue of *less* vs. *fewer*. Some uncompromisingly brand such forms as *less mistakes* as incorrect, while others note that though commonly condemned they are often used by speakers of Standard English. Before the Early Modern English period (beginning around 1500) *more* was restricted to non-count NPs with *moe* used as the comparative of *many*. At that time *less* was used along with *fewer* for count NPs, but came to be stigmatised and quite rare in this use: it is only within the last generation or so that it has become frequent. The current revival seems inexorable, given the strong pressure of analogy with *more*.]


  [1]: http://en.wikipedia.org/wiki/Fewer_vs._less
  [2]: http://itre.cis.upenn.edu/~myl/languagelog/archives/003775.html
  [3]: http://books.google.com/books?id=2yJusP0vrdgC&lpg=PA592&pg=PA593#v=onepage&q&f=false                      
  [4]: https://books.google.com/books?id=9iYusfpmmR0C&dq=Reflections%20on%20the%20English%20Language%20Robert%20Baker&pg=PA55#v=onepage&q&f=false