What not to say...

[pinot noir]

Thanks for the offer, but I'm waiting to hear back from (top school).

Um, if all else fails I'll get back to you.

Here's what I actually said: Uh, I'm not quite ready to make a decision yet (is that really bad?).

[wtncffts]

That's about right. There's nothing wrong with that, as it's true and the reasonable thing to do. You want to have as much information as possible before making such a big decision.

[anzu]

Just tell them thanks for the offer and you will let them know your decision before the deadline.

[communications13]

It is no big secret or surprise to any school that you applied to other schools. It sounds like you handled it fine.

Applying to school is like dating, you have to find who wants you and who you want and find out what traits they have that compliment yours before settling into a long term committed relationship. Universities know that, they're just eager to put the ring on but will wait b/c by accepting you they think you're worth it.