I think I know the spot Lew is speaking of just north of Castleton-on-hudson. If you look at this G-map(
http://www.gmap-pedometer.com/?r=1360299 ) you should see it just after and to the right of the 3mile marker. The spots I know in that area are on Staats Island where I use to ride the trails with a local bike club some years ago. The first bivouac spot may or may not have a bridge there now, the second one is a wildlife preserve on Staats Island road, you aren't supposed to camp there but if you are late in- and early-out you should be ok. If Lew's spot is still there I would pick it over the two I mentioned ( less bugs).
There is some stealth camping opportunities along the Albany bike path. The spot Lew menitioned is a possiblity, there are homeless and druggies around on warm summer nights so be careful. farther up the path you will go under the second highway bridge and up a small hill. just to the right in that patch of woods would be my choice. You are much better off if you can ride 5 miles further to Waterford where you can camp legally and safely in the Canal Park, ( check in with the lock master at the Erie Canal visitor center so the local fuzz won't harass you in the middle of the night)