Chris/Nanogians,
Block hijacking appears pretty popular nowadays.
yes 
One member of our local exchange had a request to advertise this unused
block: 160.122.224.0/20. It appears it was also a UBE related hijack
attempt, which failed ofcourse.
This was a block who's contact info has been marked as invalid... so
this is likely hijacked and ARIN should be or has been working on it...
With so many defunct networks over the last 5 years, do the TLAs attempt
to recoup blocks, based on length of time out of the global routing table?
I do not believe that this is the case, but I'm not working for ARIN and
don't know for sure
Any other systems in the works/place to expire invalid blocks quicker?
Ask ARIN ? I really don't know.