I came across an issue with the IE9 RC when browsing SharePoint 2010 (Oct. 2010 CU). I have to admit that there were more issues during the beta (of IE9) which are now gone. The issue I found so far is when downloading files. They simply seem to fail entirely. The workaround to that is to set IE9 to work in IE8/IE8 Standards mode instead of IE9 Compatibility Mode/IE8 Standards which is the default. You can do that quickly by changing this options in the developer toolbar (F12 key) or using the meta tag in your master page or as a response header in IIS:

X-UA-Compatible: IE=IE8

(apologies for not providing the full tag but WordPress wouldn’t allow me to paste it in)

Note that you must specify a filename again as the HTML encoded version of the view form URL is provided by default instead of the actual filename in SharePoint.