But the problem is that the Google adsense spider won't be able to get to those locked pages, and therefore won't know the content and so the ads which will be displayed will be public service or not relevant. I think it's pointless to have non-relevant ads, because you won't get many (if any) clicks. You'd be better off putting affiliate ads (like amazon) on the password protected pages, because you can control the relevancy of those ads.
Thanks for all the tips. I'll probably end up testing some adsense on both the public and user pages. dpak, I like your idea of affiliate ads in the user area. I may experiment with creating a login id for google to access those pages. ie. as long as ?id=193234 in the address bar, then the user won't get bounced back..then the google bot will be able to make it to all the user pages. Is this a reasonable idea?