Still waiting for my SPOT to arrive, so can't help you out with personal experience of placement.
However, from freinds that already have them, I have been told, that directly on the tank is not good, because there is too much metal absorbing the signal.
Also, you need to keep it away from other GPS or RF transmitting devices.
A bloke I know keeps it under the seat on his 1400GTR, and reckons it works fine from there.
Keep us updated, I'll be really interested to here of your experiences, particularly once my unit arrives.