Whilst its not the built in app, the Wireless tether app by Muller Lemons and Buxton appears to work on Murik's ROM OK.
I also have installed the Open Garden Wifi hotspot app. That gives all the appearance of working but I can't get any data through it.
The former definitely works on the Vodafone network here in Greece. I imagine it will work with others. If not try the latter option - I cannot see any reason as to why its not working - in fact it thinks it is.
LG GT540 almost certainly running a newer version of the operating system than 2.3.3 by the time you read this, possibly at as high as 900 MHz but more often at 806.