Unlike Hotmail when an email is sent from a GMail account there is no awful spam at the bottom of the email saying 'If you want to try GMail go to http://blahblah.com
- now!' etc.. One of the points I like about GMail.
GMail is also free to sign up for, no 'Extra Space' scam either. I think the only money Google is likely to make out of this is the one row of adverts down the side of the GMail account page that are designed to match the criteria of the email (personally I don't see that properly working, but I don't take any notice of them anyway).
Unfortunately, there are people who are sort of forced into having these free web mail services either temporarily or permanently. Outlook Express not working for example (and other email programs like Eudora).
As for the invites GMail is still in a "beta stage" of testing. Partly I think this is true but the whole invite thing has made people want GMail even more just for the sake of having one, so it's a clever marketing ploy (that they are not getting any money out of, cept the Google ads). So to get a GMail account you need to be invited, and these invites are put in accounts at a seemingly random basis. Later on Google will open GMail to the public.