Jonathan is correct - this is classed as VDB (Voluntary Denied Boarding) - all airlines will offer this first in an oversold situation before then going into IDB (Involuntary Denied Boarding) - each will involve a form of payment in the format of DBC (Denied Boarding Compensation)
VS will compensate a passenger for VDB in the form of a 'Select' voucher - these are good for 1 free return trip on any VS operated route (excludign codeshares) or 50,000 FC miles. If I recall Slect vouchers are also dished out for involuntary downgrades from a higher cabin (and of course the difference in fare is refunded). If you do a search on 'Select' you will see a few other threads where I have put some detail on them.
IDB is a pain, and this is where some airlines try and fob you off - like our chums over at Continental that tried it on with me at EWR last year - much to their downfall, I've never flown with them since, and boy did they wish they had IDB'd someone else - no one likes a smart alec and you can imagine the field day I had (especially when working out the fare sector value in about 5 second sjust by reading the paper ticket

)
It all varies per airline to be honest, I got offered £150 in BMI vounchers or £75 in cash for taking a flight an hour later due to an oversell to CDG earlier in the year - hmm, tough one there eh? Another hour sat reading newspapers in the lounge and being paid for it? That was for VDB, whilst our monkey like friends at KLM offered less for IDB - it was £50 or Û75 for a real inconvenience.
My take is for IDB, never ever accept their first offer as they will push it up - but in the US there are maximums that they can go to by law, so in this case remember you could always ask for the upgrade....

If you are really interested search for 'Denied Boarding Compensation' on your preferred search engine, some airlines publish their compensation policy upfront.
Take a look at
http://www.dft.gov.uk/stellent/groups/d ... 030483.pdf for info on the new rules coming into play next February for the UK and EU.
Thanks,
Richard