RB you probably won't consider using Android but if you ever planned to spend time away with a laptop or iPad then an Android running v2.2 could act as a wi-fi hotspot (and it could multitask, handle Flash, etc).
v2.2 has only just been announced but hopefully O2 will sometime offer an "unlimited" data deal for Android phones as well as iPhones. Not sure that any of the others should use "unlimited".
Maybe time for a Gadget Show / Which? / Ofcom clampdown on misleading info since most do "Fair Usage Policy" not "unlimited" AFAIK. Some have hefty over-quota fees (100+ per GB for one, and up to double that on another network)