It's not that. The issue is that the European Hero only has an HSPA 900/2100 antenna. In order to use AT&T's 3G you need an HSPA 850/1900 antenna. This is a hardware limitation, so no software update will ever change this.
There is, however, a Canadian Hero on Telus which uses the same 3G bands as AT&T. If you have one of those, it will automatically jump on the 3G network any time it's available.