General Motors bonds are now rated CCC by one credit reporting agency. This makes the company one of the least credit worthy publicly held companies still in business, and indicate that its bonds are high risk junk bonds. General Motors bonds had a less severe junk bond rating before the credit rating downgrade. More details are available here.
General Motors spend good money during the Olympics marketing vehicles like the Chevy Volt that it doesn't actually sell yet, but it may not still be in busines by 2010 when it is scheduled to be for sale. Chrysler also has an electric vehicle scheduled to enter production in 2010.
Lehman Brothers bonds were given a similar bond rating on September 18, 2008. No one expects GM to collapse as quickly as Lehman did, but there are real concerns about GM's ability to survive for the next twelve months without filing for bankruptcy.
UPDATE: More current analysis here.