I just did this back in January.
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I would do a Google search, as well as check out the Google Group for Google Apps. THere are a few ways to do it, and you can pick what you like.
THey way I did it was I had my Google Apps account (myname@mydomain.com) check my gmail account (myname@gmail.com). Here are instructions. They are technically for a regular gmail account, but it works the same way for a google apps account.
Every e-mail I ever had was brought over. It took several hours to transfer, because it will only do a few hundred e-mails at a time. Unfortunately, none of my labels were brought over. All my e-mails were dumped into my inbox without any labels. And I drank the whole “Don’t delete, just archive” kool aid that gmail served, so I had a lot of e-mails, and most of them were trash. It took me about 4 hours one Saturday to get rid of the 95% of e-mails that I should have deleted to begin with. Then I archived only the e-mails I really wanted to keep. That took me about a week sorting through several each time I checked my e-mail.
A lot of people complained about the loss of labels, so perhaps google has changed something. If they have not, and you decide to go this route, make sure all your messages from the old account are labeled coming to the new account. It will make the sorting easier. Also, as new messages go to the old account, you will still get them and be able to notify the sender of your new e-mail address.
You didn’t ask, but in case you didn’t know you can export you contacts from the old account and import them to the new account fairly easily.
One more thing. Google delays no product and feature launches to Google Apps. The reason is that they want Google Apps to be more stable for corporate users. If you want new features or products rolled out early, you will need to go to the domain setting tab of your domain management screen. There will be a check box to enable new features, and another to enable new products. I checked them both.
Hope this helps