Yes, Safari is VERY unstable on the iPhone. It'll crash often if 1) loading a page with a lot of picture elements, 2) load multiple pages, 3) use it for a prolonged period (like 15 minutes). There are many people in the forums that comment on safari crashes.
my safari never crashed using wi-fi. today i was on EDGE at Border's and crashed 2 times. others apps is fine.
the best thing to do i think is turn the iphone off and on. ( using the sleep button, just hold for some seconds for those who dont know.. i didnt until some hours ago )