Nils please take a look to see if this is an easy fix. If it looks like it will be a long fix then it would be a good idea to target it for 2.0.0.

After investigating this it looks like it could be a hard one to debug.

Each of the four NiftyCheck(...) calls that are added to the page via a partial-update when navigating to the rounded corners example cause a JS parse error.

"Parse error" for Safari, "Invalid character" for IE8 and "Unexpected token ILLEGAL" on Chrome. All of them at different, seemingly random line numbers.

Seeing as nifty.js causes JSLint to explode, it's possible there is some character in there that is causing a problem, but you'd expect that the same error would be raised by the parsing done during the refreshed load.

I'll get nifty.js to pass JSLint cleanly to see if that helps, and if that doesn't solve the problem, it must be caused by some side effect of the partial update.

Cleaning up nifty had no effect so I suspect the problem stems from an ajax issue similar to what is being discussed here:
http://osdir.com/ml/lang.javascript.prototype.window/2006-12/msg00045.html

Following the errors caused when Nifty.js fails, manually running the JS that adds the rounded corners, works as expected.

Ex: Rounded("div#outerTop","tr","#FFF","#94AECE");

Thus a callback function or other method of delaying this call until after the partial-update is completed is essential.