XID wraparound and busy databases

I was talking to someone at LinuxWorld and they mentioned they often
have activity of 6k SELECTs per second, and that they were needing to
autovacuum every few days because of xid wraparound.

I did some calculations and found that:
> 6000 * 60 * 60 * 24           518400000

or 500 million xids per day, confirming they would need the autovacuum
to run every few days.

Is enlarging the xid field something we should consider for 8.4?  Is the
autovacuum I/O less then the extra I/O needed for an expanded xid fields
on every row?

--  Bruce Momjian  <bruce@momjian.us>          http://momjian.us EnterpriseDB
http://www.enterprisedb.com
 + If your life is a hard drive, Christ can be your backup. +