If you want to try to make Opera Mini 5 Beta the default browser you can do it (kind of) with JZ SmartMort
http://forum.xda-developers.com/showthread.php?t=382043
Since this beta doesn't properly accept a url input, he had to make a workaround.
Basically, it'll copy the link into the clipboard and open Opera Mini and then you have to paste in the link. Not a horrible solution.
He hasn't made that version public, but you can PM him there for a link to a version that supports Opera Mini WM.