what a googly day!
On the same day I finally got invited to join Google Pages, I also was accepted into the beta test for Gmail for your domain. Google is now master of my domain*! Hahaha.
I can have up to 10 users on my domain for free. The interface they use when they log in looks just like GMail except is has user @mydomain at the top instead of @gmail.com. Curiously, my domain users seems to be allocated 2048MB of storage space instead of the ~2707MB allocated to GMail subscribers.
Having gmail power your own domain email is an attractive proposition. I can't see doing it for my domain just because I like having flexibility in configuring things and having unlimited numbers of accounts. But for small groups/families/business it could be very nice.
I wonder what the pricing will be like when it graduates from beta. Perhaps they will continue to offer up to 10 accts for free? They wouldn't be able to charge much for 10 users since you can get that from a lot of hosting companies for $5-$10 a month. I'm guessing it will be free up to 10, and they will start charging from there. It seems not worth the overhead to bother billing for such a little amount.
*I'm using it with a test domain I registered a while back.

