the update will not actually fail if you are rooted, although it may report failing.
if it actually fails, you can obviously see it doesnt go through the process, then there is some other factor causing it to fail.
some things that would cause it to fail all together are: non stock recovery, freezing any bloat, removing any bloat, pretty much any modifications to the stock software other than rooting will cause it to fail.
once you have any system deficiencies fixed, use voodoo from the market to protect root and temp unroot before taking the update.
if you cant figure out what you did to the system to fix it, you may want to re-flash using the proper utility for your current system, re-root, protect and temp unroot through voodoo, then try the update again.
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