I have installed the most recent OS from Apple, OS X 10.8 “Mountain Lion”, on my desktop. I am delaying installing on my laptop until the reported battery problems are solved (supposedly with the upcoming 10.8.2 release).
The biggest change for IDL is that Mountain Lion does not include X11, instead you need to download X11 from the open source XQuartz project (launching IDL will direct you to an Apple webpage which gives the XQuartz link). Early Mountain Lion IDL users on the newsgroup suggested using that XQuartz 2.7.2 could cause problems for IDL, use 2.7.1 instead. In my cursory use of the current release, 2.7.3, graphics seem fine.
Some IDL users on the newsgroup also have reported problems with license files, but I didn’t experience issues with my floating network license.
I didn’t have to install IDL on my desktop, running the installer can actually be the biggest problem using IDL on an OS that comes out after the IDL release. But since Lion removed Rosetta support and IDL 8.2 was released after Lion came out, I am guessing that IDL 8.2 should install on Mountain Lion (but I wouldn’t remove my working IDL 8.2 install!).
By the way, don’t believe the installation time. I had the same issue I complained about when installing Lion, i.e., progress stopped for a long time (about 30 minutes this time), but continued on seemingly fine.