I'm not certain you will be able to send messages w/ BB Messenger, but I know you will be able to receive. It look like the lack ofa PIN (part of the RIM OS) is the limiting factor. Please take a peek at the following threads:
My new phone is an ATT Tilt using Windows Mobile 6. It has a ton of features, including something called Blackberry Connect. I am looking to utilize Blackberry Messenger on my phone and have not been able to figure out if there is a way to do so. If I would need to install 3rd party software, I would prefer to do it from a reputable site like www.downloads.com. Thank you all for your help.