i have two nikon-canon adapters - a no-name adapter, and a fotodiox brand adapter. i found that i couldn't *quite* achieve infinity focus with the no name brand, but focus is spot-on with the fotodiox adapter (focus confirmed with a bahtinov mask) with all three of my nikon AI prime lenses.
the five most recent shots on my astrobin page were taken using these nikon lenses + fotodiox adapter with my 400D.
fotodiox adapter link