Turns out odd bits of what I thought were plastic when putting my N900 back together are aerials.

http://talk.maemo.org/showthread.php?t=80339

I didn't screw them in properly and when I messed up my USB & switch to an external battery charger, swapping batteries dislodged what shouldn't have worked very well in the first place.

This poor reception led to another problem -- the batteries drained in 2-3 hours. I blamed the charger (which may have actually worked) and eventually acquired Nokia's own DT-33. The batteries still didn't work so bought a couple of Pisens which didn't work much better.

Anyway, tightened the screws now and am on full bars. Hopefully, the batteries will start lasting a couple of days rather than hours now.