According to the book Eats, Shoots and Leaves [an excellent book, by the way, which I recommend to all who love the English language], the semicolon was first used by Aldus Manutius in the 15th century (illustration at left; image credit to Auburn University).
Now, 500 years later, an article in Slate raises concerns about the imminent death of this punctuation mark: "A 1995 study tallying punctuation in period texts found a stunning drop in semicolon usage between the 18th and 19th centuries, from 68.1 semicolons per thousand words to just 17.7."
A steep drop in semicolon usage in the mid-19th century has been attributed to the advent of the telegraph - the "Victorian internet" - because punctuation marks were billed at the same rate as words. The 20th century has seen a shift toward more concise writing, culminating in the travesty of text messaging.
I'm a great fan of the semicolon (even though Kurt Vonnegut would say that all it shows is that I went to college), so before it disappears I'll offer this little tidbit from the 1737 guide Bibliotheca Technologica which explains how the semicolon is used to guide cadence during speech: "The comma (,) which stops the voice while you tell [count] one. The Semicolon (;) pauseth while you tell two. The Colon (:) while you tell three; and then period, or full stop (.) while you tell four."
Reposted from 2008 to add the observation that computers seem to hate semicolons:
When I was leaving instructions for USPS to pick up a package for mailing, I reflexly employed a semicolon. I find it interesting that in view of all the other acceptable symbols, the semicolon is banned. Perhaps it serves some function in computer language that would lead to glitches in text transmission.
Addendum: several readers have suggested that banning semicolons may help prevent malicious "code injection" into websites. Interesting.