I've got a T-Mobile Dell Venue Pro and requested an unlock code as I'm heading overseas in the near future. Thankfully I had a non T-Mobile SIM card handy so I went and put in the provided unlock code which worked. Unfortunately, once unlocked, I received an error saying "Invalid SIM". I've tried several SIMs I have from other carriers, and each one does this. My T-Mobile SIMs work correctly.
I need a fix for this before I head out next month, and searches haven't supplied any solutions. It looks though like I am NOT the only person with this issue.