On my 64 bit Vista IDEA can open both Firefox and IE, but trying to open Safari or Opera results in an error dialog
Both are installed to the default location:
C:\Program Files (x86)\Opera\opera.exe
and
C:\Program Files (x86)\Safari\Safari.exe
IDEA more or less succeeds after I added "C:\Program Files (x86)\Opera\;C:\Program Files (x86)\Safari\" to the PATH environment variable, but there still is one problem with Safari:
Safari cannot load the file at the location specified by IDEA:
file://D:/projekte/html-prototypes/box/box-plain.html
Safari needs the URL to start with "file:///" (three slashes).
IMHO you should also add "Google Chrome" - it's a pretty easy guess that it will soon be more widespread than Opera, and probably also Safari.
My apologies if these issues were known to you anyway and planned to deal with. There have been no changes (that I could see) for some time, so I thought I post this issue.