The Yahoo! Buzz log has a post from Wednesday I just came across which lists the most common words that "vex" Yahoo!'s searchers:

Wallmart (Wal-Mart), Rachel Ray (Rachael Ray), Amtrack (Amtrak), Hillary Duff (Hilary Duff), Katherine McPhee (Katharine McPhee), Britany Spears (Britney Spears), Geneology (Genealogy), Jaime Pressley (Jaime Pressly), Volkswagon (Volkswagen), Wikepedia (Wikipedia), William Sonoma (Williams-Sonoma), Tatoo (Tattoo), Travelosity (Travelocity), Elliot Yamin (Elliott Yamin), Kiera Knightley (Keira Knightley), Kelly Pickler (Kellie Pickler), Brittney Spears (Britney Spears), Avril Lavinge (Avril Lavigne), Rianna (Rihanna), Jordan Sparks (Jordin Sparks)

 Using misspellings in search marketing campaigns (and even search engine optimization campaigns) is a very savvy use of your time.

So how do you come up with these misspellings? By far the best (at least most thorough) tool I've seen to generate typos and misspelling is Aaron Wall's Keyword Typo Generator at SEOBook. There are a few other good options including TypoGenerator.net and the SEOChat Keyword Typo Generator. The advantage of using misspellings in your promotional keywords is that they are typically cheaper to buy (SEM) and much less competitive (SEO) to achieve rankings for.