Google just announced the "Google Apps Email Migration API" for Premier, Education and Partner versions of Google Apps (note the lack of "standard accounts" in that listing.)

Using the API, programmers can import E-mail from a vast, vast array of E-mail clients out there into the Gmail application of Google Apps.

Along with the API release from Google comes a new tool from LimitNone for you non-programmers currently using Microsoft Outlook.  gMOVE: Outlook Migration allows users to pick-and-choose Outlook E-mails (either individual folders or your entire folder structure) to be imported into your Gmail account.  This goes for Gmail OR Google Apps - including standard Google Apps accounts.  Beautifully priced at $19.99 (limited time only,) this tool could save you loads of time and frustration.

LimitNone also offers a separate tool, gXFER, for transferring Gmail account settings including E-mail, contacts and calendar entries to your Google Apps account.  This particular tool is absolutely free.

Google’s API Announcement:
http://google-code-updates.blogspot.com/2007/11/each-and-every-email.html