It seems like Amp'd Mobile customers received a good-bye message earlier today, indicating that the carrier may be shutting down its operation no later than 12:01 a.m. Tuesday morning. Even though the company filed for bankruptcy protection in June, it looks like the end is nigh for the MVNO targeting the young and urban. There's a very handy Q&A page on Amp'd Mobile's site that lists options for existing customers (How do I port my number, and so forth). So if you're an Amp'd Mobile customer, you should definitely check that page out as to your cell phone options once the company goes under.
Update: It looks like Amp'd Mobile has pushed back its date to July 31st.