Nextel's "Direct Talk" feature reportedly allows off-network communications, but I cannot confirm the reliability of this operation first-hand. I also think you have to manually select this function, the radio does not automatically revert to it.

I have also heard anecdotal evidence that Nextel DirectConnect seems to get through when the networks are congested. Presumably due to the smaller amounts of data going through, and it is more "packetized" in nature.