apt-get update -> Dynamic MMap ran out of room

[Dave <dibbler@frii.com>]

Recently whenever I try to "apt-get update" on my workstation, it gets
all of the package lists then runs out of memory trying to read them. 
At least, that's how I interpret the tail end of the output:

Reading Package Lists... Error!
E: Dynamic MMap ran out of room
E: Error occured while processing yelp (NewVersion1)
E: Problem with MergeList /var/lib/dpkg/status
E: The package lists or status file could not be parsed or opened.

My system has 512MB of RAM, most of which should be free, and plenty of
HD space.  I also have 3 other Debian systems which don't have this
problem, all pointing to the same apt-proxy.  I'm also pretty sure the
package list and status files are fine, but I don't know how to check
for certain.  dpkg works fine, but I can't apt-get anything until this
is straightened out.