However, I received another email just now with this important information:
As of May 10, any organisation that signs up for a new account will be required to use the paid Google Apps for Business product in order to create more than 10 users. We honor our commitment to all existing customers and will allow you to add more than 10 users to your account for [domain name] at no additional charge, based on the limit in place when you joined us.
I am using the Google Apps Standard Edition, in case you're wondering. This is basically the free service. At 10 users, it is still great value considering that it is free (I believe the original limit is 50). But even better is the fact that those who signed up before May 10 will still get the original quota!
If you own a Google Apps account, you may start transitioning your users. In the week of May 12, Google will automatically do it for you.
I plan to do mine this weekend.