I would imagine that things would be a lot faster over WiFi than over GPRS, based on a couple of things:
o The relative speed of web browsing on a Blackberry connected to the "Blackberry Device Manager" on a PC via a USB cable, compared with the same Blackberry over the air.
o The relative speed of browsing via WiFi on a WM5 device such as an HTC Universal compared with via GPRS on the same device.
RIM have sold WiFi-only Blackberries in the past, so it's not entirely new to them.
From a business point of view I can see the advantage of having one device that does email and is both a work phone extension when in the office and a mobile when out of it - switching automatically between the two. VOIP on a phone handset would also be useful.
Obviously the mobile carriers are unlikely to welcome moving to an "everything IP" scenario, but it's happening in the landline business so I can't imagine they can resist forever.
With regard to "still need a data plan", with push email on a WM5 device I think that you actually need to be on GPRS to receive email, although how much data is transferred on GPRS and how much on WiFi (if you have a connection on both) I don't know. I've not found devices that support both to be particularly intelligent as to which to use.
The screen's still too small for "normal web browsing" though, although for occasional updates about e.g. just how badly England are losing to South Africa at cricket, it works pretty well.