From what I can tell, this was caused by AppleVerticalScrollbar's computeTrackLength method sometimes returning NaN which totally throws off all the calculations based on that and it eventually trys to set NaN as the scroll bar height in the _setObjectLength method (which WebKit didn't care about before the change).
My patch overrides the _setObjectLength method and checks if length is a number before setting it which fixes it (on Tiger at least, don't know about what changes have been made to this on Leopard, though I doubt this breaks anything).
Created attachment 12054 [details]
This fix would probably be better off in inspector.js with a note about why its there wouldn't it? I'll leave it up for comments on the logic and upload a new patch in the morning..
Matt, can you file a bug in radar at http://bugreporter.apple.com/ mentioning the breakage that this covers so that it can be tracked internally by Apple?
Filed as radar 4901491
Created attachment 12061 [details]
Landed in r18444.