Actually, you can make IMAP work with a BES server, but you need to get BES Hosting that supports it. It's the cheapest way to get BES, about $10 per month to purchase BES Hosting and a Microsoft Exchange email mailbox address at http://www.myblackberry.us
Before buying the Treo, please make sure you educate yourself on the advantages vs disadvantages of the Treo. You might quickly end up being willing to pay more once balancing out the advantages. It depends on what you are ultimately after.
Secondly, THERE ARE THIRD PARTY EMAIL CLIENTS for the cellphone-based BlackBerry (but you still need to pay for the BlackBerry plan and some of these requires MDS, which begin at about $10 per month to enable the TCP/IP POP connections directly from the BlackBerry.). It's called Reqwireless Emailviewer, there's Hotviewer as well (HOTMAIL client)
Sempai, it may be true that Treo600 may make this person happier in particular but there were workarounds you did not know about. Please be careful. Please do not do it again without first providing the extra information I just gave. There are tradeoffs yes. Thank You.
My 7280 with BlackBerryOS 4.0 has categories, has the same email folder tree as Microsoft Exchange (Select folder feature), and it is my own opinion that 4.0 OS now has a superior PIM to both my old Palm and my iPaq 4155 now the PIM features is parity and I am able to enter PIM data faster with the thumb keyboard.
For more information about BES Hosting, or Email Methods, and Blackberry vs. Treo, please see my BlackBerry FAQ