[Tailor]

Hi gibodean,
1. Your SIM will be fine for WiFi, probably fine for 3G too, there was supposedly a change a few years ago where the SIM would actually have 3G on it but that's all been superseded now, if you want to be sure, put it in a 3G capable phone and verify you get U or 3G or UMTS or whatever the phone shows it as!
2. WiFi works for customers with unlimited blackberry tariffs, it's not supported on PAYU Data. It didn't work with BIS Unlimited but that's sorted now.
3. WiFi will always connect and allow you to browse the web (if you select the right browser!), with PAYU Data that’s about it, with Unlimited data tariffs then a connection is also made to RIM's servers over WiFi and thus that traffic also uses the WiFi, this would include the various messengers, RIM's websites, email, etc
Advantages being speed and free data whilst roaming.
On the Pearl 8120 then you also get the benefit of having data/voice simultaneously, however 3G also offers this so the Bold effectively has this anyway.
O2 doesn't support UMA.
4. I'm not a big follower of apps so get some more input on this one, I thought YouTube was quite cool and there is Telnet and VNC apps available.
I'm not aware of any VoIP clients, certainly not any free ones.

The other feature I use WiFi for is BES, RIM offer one free licence with the professional version and you can connect wherever there is WiFi so i get the advantage of Sync'ing Contacts, notes, other email folders.