GMAC is the financing division of General Motors. GMAC Bank was an FDIC insured online bank account. You can see how it would be troubling to see that GMAC is in trouble.
Today GMAC announced that it's taking $7.5 billion in federal aid to stay afloat. However, accounts at Ally (formerly GMAC bank) should be unaffected. As long as you're under FDIC insurance limits, your money is safe. Of course, total failure could result in inconveniences in getting your money, but this is not currently expected.
Ally launched a publicity campaign to spread the word, spinning the bank as a transparent, consumer-friendly institution. I just noticed that they're advertising here at About.com as I began to write this.
For existing customers, routing numbers for ACH transfers and checks should work until November 1st 2009. After that you'll need to use a new routing number.
Further reading:

