I commute about 100 miles a day - yesterday I noticed a squealing/screeching sound coming from under the hood.

It was louder when I accelerated, and I could also hear it when the car was idle in a parking lot.

The car's got about 70,000 miles on it.

What could this be?

 

sounds like a loose serpentine belt to me.

 

Agreed... Check the tension and condition of the belt...
Change if needed which is most likely if its been slipping.
Look for tensioner pulley.. U usually put a wrench on it to
Move it which allows u to remove and replace the belt.. Haven't done one on the xD, but typically an easy 10 minute job..

 

It's probably the water pump. Check to see if there's any red residue near the pump. Doesn't necessarily have to look wet (mine didn't). That's my bet.
Service bulletin number is S-SB-0048-09

 

Since this is an 08 and many other 08's lost their pump around this mileage..
you are probably correct..
Their is a weep hole on the pump.. they usually leak some through this hole and it's a good sign of the pump going out..