Tempe is incredibly lucky to have so many great Thai restaurants. There are at least four within a mile or so of ASU. Most are popular enough that you'll be waiting for a table on any given weekend around 7pm.
For some reason, Tempe's first Thai restaurant (according to the comment on their menu) is tucked away in an unpopular strip mall off the 101 next to a DES office. My girlfriend and I stopped in without calling first on a Friday night and had the place almost to ourselves. The tom ka gai was nicely spiced with lemongrass and lime leaves, and didn't have too much coconut milk. Our green curry was spicy and had lots of fresh Thai basil. For dessert we had a frighteningly tasty coconut custard dish with sweet sticky rice.
Siamese Cat isn't as posh, trendy, or expensive as a lot of other Thai places, but it's a great alternative to the crowds if you're looking for an exotic, romantic weekend dinner.