Aptitude has locking problem on cache file -- cannot upgrade to latest sarge.

[Hendrik Boom <hendrik@pooq.com>]

Yesterday (2004 jan 12) , when I tried to upgrade to the latest sarge, 
it  spend a marry 23 minutes downloading all the changes, but when I got 
around to telling it to go ahead and install, all it did was give me the 
message
                                 Apt errers
  W: Warning: could not lock the cache file.  Opening in read-only mode

I press return to say "OK" (though everything ie evidently not OK), and 
it doesn't do any installations or upgrades.

How do I get out of this wedged state?

-- hendrik