Everyone has a different opinion on this one…
Here is mine:
Advantage of multiple:
1. Can rank for keywords by tying in the domain name with your keywords
2. Can rank on MSN because that engine likes new sites (but this only works when the site is new or the pages are new)
Advantage of 1:
1. You only have to update 1 site instead of many
2. You will most likely never leave a site unattended for months on end
3. You will gain more power due to the increasing size of the site
4. You will get more links because the power of your site will increase faster (most likely)
These are just a few of the adv/disadv that I have noticed.