7.3: Change in cursor behaviour?

I've been getting reports of one of my test scenarios for libpqxx
failing with postgres 7.3. At the moment I can't reproduce this (I'm
still on 7.2) and I can't find anything pertinent in CVS commit
messages, mailing lists etc. so I'd really appreciate any lucidity from
this list.

My problem appears to be related to moving a cursor backwards beyond its
point of origin. I realize this is a tacky thing to do, but I sort of
got the impression that moving backwards by some number larger than the
current position would be equivalent to MOVE BACKWARD ALL. Which makes
my code a hell of a lot simpler. It makes a good portion of my life a
hell of a lot simpler, come to think of it, so I'd really appreciate
having this property in the future.

The scenario boils down to: Create a cursor, fetch n rows, move minus 2
billion or so rows, fetch 1 row. That last fetch used to give me the
row I was hoping for (the original first row again), but with 7.3 it
appears to yield nothing.

Is this intentional? Should I change my code? Have I been a bad boy?

Jeroen