OK, its confirmed.

I switched off my blackberry's mobile network and verified that the signal is at OFF. I am still connected via my office's WIFI.

I then use my laptop's outlook to send myself an email from my BES enabled office mail to my BIS enabled gmail. 5 seconds later, my blackberry's gmail account receive the email. 30 sec later, my blackberry's BES side reported the outgoing mail in the sent folder.

I tried replying in gmail (again via laptop) and the same things happens, first the BB detects the gmail sending the reply, then (faster this time) the email shows up in my BES inbox in my BB.

I tried sending both direction using my BB, at it works too, with about 2 sec response time.

So, I can assure you, you can get your email (I havent tried the BBmessenger) without a dataplan.