Depends on your wireless router. iPhone is both 802.11b and 802.11g compatible, with .11b being 11Mbps, and 11g being 54Mbps.

Of course it will never reach those speeds as the internal processor is only 620mHz or something, and the cache is tiny, but then it's a handheld so thats unsurprising So even tho it can use that speed of wifi, it will always be held back by it's processing power - same as how old laptops etc aren't as quick to load and display web pages as new ones

But it's faster than 3G for sure... My server (which is also my router and wireless access point) is away for the moment, as it went fubar So I can't use mine on wifi at the mo, and installing apps and updates is painfully slow over the cell nets (specially since mine is the 2G model) but I discovered MacDonalds do free wifi, so I took a 3 min drive up there yesterday, did all my updates in a quarter of the time, and installed two new apps, and had the added a bonus of a cheeseburger while I waited