You will need a BES or BPS to sync your Contacts and Calendar over the air but if you want additional email addresses which is most of the reasons people want IMAP you can do it.
In fact, in the BIS setup there is a very easy way for the BlackBerry to communicate with OWA without even setting up POP or IMAP on your server. I was shocked too but when setting up a BIS BB and looking through the options I entered my email address and it autodiscovered my OWA server and added my email. However in this case it was a different users account but it does propagate both ways. So I would suggest setting up your main account and having all mail forwarded to that and having basically different Exchange accounts which you could add onto BIS that don't recieve email but you can use as different "From" addresses.
The question now is "why not" a BES or BPS at least. With one user you can do it for absolutely free besides the Server OS licensing and hardware (but there are always VMs). Exchange 2007 SP2 supports BES 4.1.6 and BPS. This page
lets you enter in your BlackBerry PIN and receive a free license and download of the software. A trick too which I don't know if it is legally covered by RIM is that you can also enter in a second pin and get another license. Take the "bescal" of that and add it too your BPS and voila you get now two licenses. So you could theoretically do it with all users. However, I don't know if this is legally supported by RIM so maybe stay on the safe side and buy additional CALs.