There seems to be some issue with the recently released beta of ext-core as well as the latest from svn that makes the file not play "nice" with Visual Studio.
If you include the ext-core.js or ext-core-debug.js in the body of html document IE7 gives a "line 1008: _load is null and not or not an object error". If you include the script in the head you get an error in the Visual Studio IDE that makes intellisense fail completely in any place that the file is referenced.
"Warning 1 Error updating JScript IntelliSense: C:\TFS\VS2008\Shapes\Shapes\js\ext-core-debug.js: '_load' is null or not an object @ 1007:20 C:\TFS\VS2008\Shapes\Shapes\Default.aspx 1 1 Shapes"
Oh yeah, I don't use Visual Studio, but I can't get any Ext3 code to work on IE without commenting out that line. I never ported my changes to onReady over since the code is different. I'd like not to... still have to fix mask first, higher priority.