A quick googled showed that this question was asked a few times in 2007 and 2008 and I was hoping there was better news now that we're 4 years on!
When adding contacts, is there a way to change the default of Work phone to Mobile? It currently always adds as work but i use it as a personal phone and would love to change this to mobile.
Thanks!!

Nope. Not natively.

I believe there might be a third party application that will do this.

Otherwise, just cut and paste them from one line to the other.