my SENT mail does not show up in my iphone

What is the account type - POP, IMAP, or Exchange?

IF you are talking about Exchange on Google with Gmail (ie chose exchange and using m.google.com) here is what I did to fix it.

Using your browser, sign into Gmail. IF, NEAR THE BOTTOM of your folder lists, you have a label(folder) on the left side called "Sent Items" then delete it (mine never had anything in it and i think it's for conversations.) BTW, I KNOW this is a Junk E-Mail (remember i already deleted my folder) but wanted to show you the drop down and Remove Label option.

Then go to your Settings -> Mail, Contacts, Calendars -> Exchange -> Mail Folders to Push -> and put a checkbox next to SENT (looks like a paper airplane)

backout, go back into mail -> Exchange -> and you should then see the same paper airplane there.

What was happening with mine was it would show up and then disappear as soon as it synced. The only thing i can think of is that GMAIL has a folder called SENT xxxxx (that I deleted above) that confused the iPhone and i believe it's for Messages not Mail (via google talk)

Once that was deleted, Voila! No more missing sent mail. Also, it has showed up on my iPad again.

Hope this helps.