I'm adding this as an answer because it's too long to fit as a comment... The odd thing to me about gonna, gotta, and wanna (and their close relative hafta, and their more distant relative gimme) isn't that these words have become mainstream in both spoken and informal written American English; it's that the Merriam-Webster's Collegiate Dictionary series refuses to acknowledge their existence. The Eleventh Collegiate does include entries for two related nouns—gimme and wannabe—but it doesn't provide entries for the common contractions from which those terms arose.
Ultimately, if one purpose of a dictionary is to identify and define commonly used words in a language, Merriam-Webster's diffidence toward this class of contractions is hard to explain. In contrast, the Encarta World English Dictionary (1999), the American Heritage Dictionary of the English Language, Fourth Edition (2000) and the New Oxford American Dictionary (2001) include entries for four of the five contractions I've mentioned (the exception in all three cases is hafta), although they don't agree entirely about the status of those four words.
Encarta calls gimme, gonna, and wanna "nonstandard," but characterizes gotta as "informal." For its part, the New Oxford American lists gonna and wanna simply as "Informal," but it says that gimme and gotta are "Informal...(not acceptable in standard use)." The Fourth American Heritage labels all four terms "informal."
Perhaps Merriam-Webster fears that, if it were to acknowledge any of these terms, it would be obliged to open its doors to an indefinite number of other common unpunctuated contractions: oughtta, coulda, woulda, shoulda, kinda, sorta, dammit, and so on. It's not a very compelling reason, though, so perhaps some other rationale lies behind the policy.