For orientation: week 7 alerted me, but changing to 2.6.24.2 helped.
Then on the last day of week 8 I rebooted the same kernel for further
investigation, and simulated memory pressure 3 times, which resulted
in the drops (down to about 1k each time).  The increase seems to be
sublinear now for some reason, though the usage pattern of the machine
is very much the same.

Week 05           : 2.6.23.14
Week 06 Feb.   -10:   -""-
Week 07 Feb. 11-17: 2.6.24 (HEAD: 25f666300625d894ebe04bac2b4b3aadb907c861)
Week 08 Feb. 18-23: 2.6.24.2
Week 09 Feb. 24-  : 2.6.24 (same as before)

So, it looks like there's no leak, but the reclaim behaviour has
changed significantly, exactly as you suspected.
-- 
Thanks for your time,
Feri.