I just burned roughly 2 hours learning that Windows 7 Ultimate 64-bit IIS (7.5) can't serve a fully loaded jquery-ui-1.8.12.custom.min.js file (which is around 208k). The only solution I've found so far was to split the file in 2 and serve them separately. This issue is NOT specific to a particular version of jQuery UI.
If you know a better solution, please do share with the rest of us.
Symptom: REALLY slow loading page which results in an error that can be found on the dev console. Using IE9 Dev tools, Network profiler, I was able to see the "pending" status of the file which wasn't getting served properly.
Solution: I used UltraEdit to break the file in 2 and in my app I load both files in the correct order so that the Core components load first (in case it matters).