All BBM would gain you besides the "BB hype ting" is free messaging to other BB owners, which you really ought to have anyway (in the way of free texts) on a phone contract worth circa £750 over 18 months!
Admittedly not with a BB, but I do the whole "carrying around a phone AND an iPod touch" and it does get a bit boring. Not only are you more likely to loose your iPod, but it'll feel like a downgrade due to the lack of communication and interrupted internet access.