From the PostGIS Listserv comes a very useful comparison table that has been partially vetted by the members of the list. I just wish they included Oracle Spatial and Oracle Locator.
/> As part of our due diligence work, we are formulating a comparison of the
/> spatial functionality offered by SQL Server 2008 Spatial, PostGIS and MySQL.
/> Its currently in draft, but we hope to release within the next week.
/> We would appreciate if people could take a quick look at it and let us know
/> if you see any gross errors or important things we left out. Most of this
/> information we curled from reading the help docs of each product and from
/> our general familiarity with the databases and GIS products out there. We
/> tried to be as fair as possible.
/> Regina and Leo