IMHO though, it's a short-term solution at best. Reason being, you have very limited control over what you do with these subdomains once they are successful. Still best overall to run your own domain for the long haul. Promoting MyCoolWidgetSite.com is a lot easier and better for a variety of reasons, than promoting MyCoolWidgetSite.someblogprovider.com. ;-)

Plus I have to say, all the good .com's are *not* gone...only the obvious ones. Create your own new take on things; be creative; make up a word that's an amalgam of a few other words you wanted. Plus, who said you must have a .com?

Worst case, go buy a domain. Some very, very cool domains, if viewed from the end-user perspective (i.e., not as a domain reseller), are still cheap in the scheme of things. We're still very early in the game.