If someone has 20 sites all on 1 shared IP and one site ends up spamming their site to much would that 1 out of 20 sites get delisted OR would ALL the shared sites be delisted from the search engines??
If it's one bad site on an IP Google probably would penalize the one domain. But if there are many bad neighborhood sites on a given IP then they sometimes will penalize all domains on the IP. It's a sort of guilt by association penalty. They simply penalize the entire IP and wait on the "good guys" to submit a request for reconsideration. They will review the site and remove the penalty for the good guys once they submit a reconsideration request, but it's a hassle I prefer not to deal with. This is why I highly recommend paying an extra $2 per month for a dedicated IP on which to place your own sites so that you can avoid the hassle of waiting on your reconsideration request to be processed should this happen to your IP.
I think it depends how severe the spam is too. For some spam Google gives a slap on the wrist while other spam can result in a search ban and pr removal. If it were me I would move that one site to its own ip. As Canoncial mentioned, if you can afford it, a dedicated ip for each site would be much safer.