Even at 140 characters, you're not safe.

Spammers have found another way to invade even Twitter, no longer with the clunky mass-following method which I've encountered a couple of times already, but now deviously exploiting hashtags and the Trending Topics feature on the sidebar. Mashable's Adam Ostrow:

How? Simply include the trending term in your tweet. Then, anyone who clicks the trending topic will see your ad, for free. Today’s example comes in the form of “Apple Shampoo,” a song from Blink 182 that is being shared aggressively today because of a tweet from band member Mark Hoppus (@markhoppus).

Here’s an example: most of the tweets below really have nothing to do with the band or the song, but are rather an ad for some sort of affiliate marketing scheme. Visiting the offending user’s account on Twitter, it’s clear that they’re simply using Twitter to push affiliate links, and now exploiting trending topics to gain more traffic.

Incidentally, I also came across this older but still very solid guide to the benefits and use and abuse of Twitter over at The Lost Art of Blogging. (Yeah, I'm months behind the curve on this one. So sue me. That's where the "curmudgeon" and "antiquarian" bits in the header come in.)