The two most widely followed style guides in mainstream U.S. publishing are The Chicago Manual of Style and The Associated Press Stylebook. They agree in endorsing the use of a period after Mr or Mrs. From AP Stylebook (2002):
Mr., Mrs. The plural of Mr. is Messrs.; the plural of Mrs. is Mmes. These abbreviated spellings apply in all uses, including direct quotations.
And from Chicago (fifteenth edition, 2003):
15.16 Social titles. Always abbreviated, whether preceding the full name or the surname only, are such social titles as the following:
Ms. Mrs. Messrs. Mr. Dr.
Publishers that follow either of these guides require a period after such social-title abbreviations, unless their house style guide overrules the standard guide on this point.
As a counterpoint, MLA Style Manual and Guide to Scholarly Publishing, second edition (1998) opposes using formal titles at all:
3.6.2 Titles of Persons In general, do not use formal titles (Mr., Mrs., Miss, Ms., Dr., Professor, Reverend) in first or subsequent references to men or women, living or dead (Churchill, not Mr. Churchill; Mead, not Professor Mead; Hess, not Dame Hess; Montagu, not Lady Montagu). [Exceptions for certain "women in history," such as Mrs. Humphrey Ward and Mme de Staël, and for certain titled nobles, such as Henry Howard, earl of Surrey, omitted.]
The tendency in British style appears to be against end punctuation of social titles. The Oxford Style Manual (2003) doesn't discuss the matter directly, but it consistently gives examples of Mr, Mrs, Dr, and the like without periods. For example:
Titles used as identification or clarification after a name normally are not capitalized, especially in US usage):
[Relevant examples:] Mr Gladstone, the prime minister; Dr Primrose, the parish vicar
And in the "Oxford Dictionary for Writers and Editors" section of The Oxford Style Manual, the entries are similarly consistent:
Dr doctor (before name)
Mr Mister. pl. Messrs
Mrs Missis, Missus (corruptions of Mistress)
Ms the title of a woman whether or not married (no point)
Oxford doesn't apply its "no point" rule across the board, however. The style manual takes the opposite approach in its treatment of military abbreviations—Cpl., Sgt., Lt., Capt., Maj., and Gen., for example—and of Jr. after proper names.