Return Path's Tom Sather explains when it's OK to use a shared IP address. Basically, when your volume is very low. He suggests that the cutoff be 50,000 messages a month -- below that level, you should be on a shared IP address. Above that, a dedicated IP address is recommended. I personally think there's some flexibility there, but overall, you do have to draw a line somewhere, and it's pretty good guidance. (I usually recommend that senders mailing to more than 100,000 recipients per month utilize a dedicated IP address.)
The one obvious question that I think goes along with this is, when is it NOT okay to use a shared IP address or shared IP address pool? Here are three scenarios where I think it is truly NOT okay to use a shared IP address or shared IP address pool.