Couldn't find on the forum where this issue was "fixed" or discussed. But, I am having the exact same problem with my Android 3 phone and the Android default email client using IMAP. Everything works okay except my biggest gripe is that it will not save a copy of emails I send from my phone anywhere! They are not on the phone -- nor on the server.
Thinking of going with K9 email client that I've heard others use because of this issue with the Sent email not being saved. I use Android 3 default email client with two of my email accounts, and Touchdown for my corporate email exchange account.
Has anyone else run into this issue -- or have a better "fix"?