They key to any mashup is an available API.  In my research in thinking about different interesting Ifbyphone mashups I have run across a couple...  Here is the list of some:

• Yahoo (http://developer.yahoo.com/) - They have a HUGE assortment of different API's to play with (and best of all they are free!).  Some really interesting ideas would include traffic lookup, look up local information, etc.  I actually used this to create a Yahoo Weather / Ifbyphone mashup.  I will post the source code in the coming days...

• FlightAware (http://flightaware.com/commercial/flightxml/) - Some real potential here for creating live flight tracking capabilities using your phone.  API is not free, however the price is very reasonable.

• FreshBooks (http://developers.freshbooks.com/overview/)  - Using their service one could easily build an automated time in/time out application where a service worker could call a number, enter in the project & job ID, and then either clock in or clock out.

• StrikeIron (http://www.strikeiron.com) - Again, lots of API's to play around with including reverse lookup capabilities.

• Yelp! (http://www.yelp.com/developers) - Great for looking up local reviews on businesses.

I am always interested in other API's out there that could create fun/useful applications.  If you come across some, please share!