I installed Snow Leopard when it came out last Friday and have been using it exclusively since then. The only thing related to IDL that has been a problem for me so far has been that the IDL 6.4 Assistant (the old online help which I still use when working from the command line) requires Rosetta, which is no longer bundled by default with Snow Leopard. The install process rolls your X11 version back a bit and a Snow Leopard update is not available from XQuartz yet, but I experienced no problems from this.
Overall, this was much smoother than Leopard, where it was months before everything was smoothly working again. After upgrading several applications that came out with Snow Leopard updates on Friday also, everything is now working as well as (and in several cases, better than) with Leopard.
UPDATE 9/13: We have had problems installing IDL 7.1 on Snow Leopard; the installer is evidently not Snow Leopard compatible.
UPDATE 9/21: Installing Rosetta should fix the IDL installer issue.
UPDATE 9/28: M. Katz reports on comp.lang.idl-pvwave that IDL 7.1 is two times faster on Snow Leopard.