I have a blog on a free wordpress.com account and its very popular and has huge traffic, but i can't put any AdSense on it because wordpress doesn't allow it. Is there any way i can transfer my wordpress domain to my own domain without loosing Google rankings and PageRank?
If you have access to the .htaccess you can 301 the pages. Actually you can do it with META refreshes or PHP scripts. I'd be more worried about the WP people discovering it and cutting you off though.
well you would want to inform the sites who link you to the existing WP change to the new url so that you have the PR and use 301 re-directs as rmccarley adviced so that the SE bots know your new URL..