to believe information already posted online, it took a mere two hours to find an exploit in the new Safari 3 beta. As this is a beta, we're not amazed that it's full of bugs, but some of them seems to be exploitable as well.
Several websites have reported about ways of hacking the new web browser and you can find some interesting information here
about what people have done to make this new browser fall apart.
How likely it is that you'll run into someone trying to hack your browser is a different matter, but as always, be careful when you're using a beta application, no matter who makes it, as it is always much easier to find ways into unfinished software.
Let's just hope that Apple will release a fix for these exploits in a more timely fashion than what it takes M$ to fix a bug in IE.