So I finally got to see Enchanted yesterday after trying to get there for about three weeks. And it was definitely worth it. I haven't left a movie feeling that good for a long time. And "That's How You Know" will be nominated for an Oscar, no doubt about it (whether it wins depends on what it's up against). I just hope they don't do another "Dreamgirls" and nominate more than one song from this movie. Not only does that lead to nomination inflation, but it splits the vote, which is (probably) why that other song won instead.
A little research (i.e. looking on Wikipedia) reveals that last year was not the first year that more than one song from one movie was nominated, and that it is possible for a movie with more than one nominated song to win. In 1983, the theme from Flashdance won, even though "She's a Maniac" was also nominated (and two songs from Yentl were nominated that year, too), and in 1989, "Under the Sea" from The Little Mermaid won, and "Kiss the Girl" was also nominated. The same thing with Aladdin. And Both Beauty and the Beast and The Lion King had three songs nominated, and they both won, so there you go.
What I was really looking for (and still haven't found) is some kind of rule about the number of nominations for Best Original Song. Because whenever I start talking about that category, my mother always brings up the 2005 awards, when there were only three nominated songs, and "It's a Hard-Knock Life for Pimps" (her name for it) won. Mostly she likes to rant about how one of the three songs from the fourth Harry Potter film should have been nominated, and probably would have won.
Now, I know that there have to be a certain number of eligible animated films in order for there to be five nominations (I think that it's over 15, but don't quote me on that), otherwise there are only three. So there must be a similiar rule for the songs, since there have been years when only three songs were nominated. But so far I haven't found anything useful.
That wasn't really why I started this post, but I'll have to get to that later.
Edit: This article goes into the Best Song Nomination Process.