I believe the success of flickr.com (registered in 2003, launched in 2004) kicked off the current vogue for these names, though there are earlier examples too:
http://www.branddialogue.com/diablogue/2006/06/names-that-hu...
Misspelled words -- especially shortened ones -- have certain persistent advantages as names for web companies. It's easier to find available domains; it's a unique word for search engines; it's short and therefore quick to type.
Of course there are disadvantages too... Twitter was originally called "Twttr" when it launched in 2006, but later restored the vowels:
http://techcrunch.com/2006/07/15/is-twttr-interesting/