I've been struggling with this for some time. I really like that the iPad and iPhone both will talk to Gmail via Activesync (Exchange). This allows me to have Mail, Contacts, and Calendars pushed / synchronized to my device from Google. But the Mail implementation is flawed because you cannot set a different reply-to email address. Actually, you *can* set a reply-to address, but it reverts to your gmail address when you launch the Mail app. I want my incoming mail to come in via Gmail, but I want my outgoing mail to be from a different address (I don't use my gmail address as an explicit email address).
So here's what I've done:
1. Set up my devices to [talk to Google via ActiveSync](http://www.google.com/support/mobile/bin/answer.py?answer=138740&topic=14252). Check sync for Contacts and Calendar, but NOT for Mail.
2. Set up a second Gmail account as "Other" IMAP -- not using the default Gmail setup ([here's why](/journal/2010/04/05/how-to-properly-set-up-gmail-with-your-ipad/)).
3. Set up the IMAP account to use my normal SMTP server (the one my echeng.com address uses).
I tested this, and it works. I get my email through Gmail, but I can send through another email address.. although with this setup, I've lost push email.
You can't win 'em all.