The court might not allow any UN people to give their opinion if such challenges occur, especially if they demonstrate the bias you and a few others in this thread have said they would. There's also the issue that foreign election monitors are barred from several states, which would only add further questions to any conclusions they draw. (not to mention the fact that the UN and OSCE have the ability to do exactly nothing with regards to our elections)
Oh, and since they haven't done it yet, here are some cites. In 2004 the UN didn't monitor the elections, but a European group did. This is the same group that will monitor the elections this year.
They ARE NOT part of the UN though. (the title should be changed to reflect that)
http://www.washingtontimes.com/news/2004/aug/6/20040806-115723-1192r/?page=all
I couldn't find anything about 2008 that wasn't linked to a grossly biased website, but there were indications that this same group was also here in a small capacity in 2008. I also wouldn't be surprised if they were here in 2000 as well.