afaik Blackberry's are the only devices that offer push email, so you'd lose that by getting an E51. But then you'd probably have working wifi ;)
Oh and I've just read this
thread, doesn't sound like Orange are any better with wifi AND you would have to start using their broadband services.
I can't believe O2 tell people to talk to their ISP. It's something their end, and the fact that some have working wifi and some don't still indicates that there is a problem which they are failing to take seriously.
For those that don't have working wifi, do you see a grayed out wifi icon? This should allow you to browse the internet on the BB browser, but you won't be able to send/receive email. That was what I found with mine, however after Tailor made great progress with O2/RIM mine is now working. That was after resending the service books.
Seems like for some of you a serious letter of complaint is in order :(