I became pg when DS was only 5 mos old, and I didn't know about the 'surprise' pregnancy until about 3 months later... to make a long story short, I continued to nurse throughout, and we tandem nurse now. DS's are 13 mos and almost 2.5 yo.
As for my supply, DS didn't start solids until he was 8.5 mos old, and I knew he was gaining and growing. I'd say toward the 3rd trimester, he'd nurse for just a minute, and that was enough so I would guess my supply was down. I'm glad he suckled, though, because I was afraid if he stopped, he wouldn't start up again once my milk was in full force.
The best feeling I had was when DH brought my 3 children, including my son mentioned above, to the hospital to see me and the new baby - all eyes were on the baby of course, but I grabbed my son and scooped him up into my lap and he nursed like crazy from then on - and still does!