Lots of people, including Joe Biden, have said that some foreigners would be challenging Obama early on. I see two candidates at the front of the pack, one a nominal ally. The first is Russia, which has arguably already done so with its announcement of moving missiles next to the Polish border in response to Poland accepting an anti-missile shield. It could be that they did not mean this as a challenge to Obama, but rather had been waiting until after the election to do this so as not to influence the election. However, this looks all too much like the half-baked thinking of Putin who somehow thinks that threatening others will make things go Russia's way. Obama had long expressed reservations about the shield, and the Poles had been resisting Bush's pressure to install it. Obama also initially had a nuanced view of the Georgian-Russian conflict. But then, Putin invaded Georgia proper. This pushed Obama to support Shaakashvili unreservedly, and also triggered Poland to accept the anti-missile shield. Putin (or more likely, Medvedev) might have been able to negotiate with Obama to withdraw the shield or put it on hold, but with this missile move there will be no way that Obama can do that. He will have to show that he can stand up to the Russian bear and support the shield. Just plain dumb on Putin's part.
The other likely challenge may come from Israel, which has been very weak in its congratulating Obama. This may be the flip side of all the enthusiasm for Obama in the Arab and Muslim world, with even President Ahmadinejad of Iran sending congratulations. The Israelis are nervous about Obama's middle name and his family background, and although he has spoken unreservedly about supporting Israel, they may feel a need to test him on this, to do something unpleasant and provocative to get him to show more openly his support and to weaken the enthusiasm for him in the Arab and Muslim worlds. The obvious move would be a strike against Iran, although there are other possibilities out there. Maybe they will not pull something like this, but at the moment, the signs look worrisome to me.