I do not think you can change the default browser. I use Opera 4.2 and it works great on my Tour. I tried the 5.0 but had some glitches. I will wait until they get the final version completed.
You can't really change the default browser. You have to open everything thru Opera manually. However, when I tried the Opera 5 beta on my phone a while back it automatically took over my phone. So that was weird. I think once they release an offical version you'll be able to set it as your default, but not the beta.