Problems with Onstar...

[f5fstop]

Might be a problem if there are limited verizon towers operating in N.O. area. The GPS signal is received by the Onstar module, and sent via the verizon phone lines to the Onstar headquarters. Onstar can ping the module to get a more precise location, but the info still travels on the phone link.
However, i would have thought if they could talk to you, they could locate you, and Katrina did not mess with the satellites.
Possibly it was due to lack of bandwidth in the phone lines. Not sure.

As long as the alarm install did not screw up factory installed components, no, it won't void the warranty. Could the installer have messed with the Onstar module, or the GPS antenna?