An excellent post over on SaaS blogs locks at two possibilities for PaaS. One is very vertical in nature (AppEngine, Force etc) providing all the various layers needed for the offering (library layer, delivery layer, compute layer). The other is much more horizontal in nature where, say, a vendor provides the compute layer but do so very broadly.
Sinclair contends that the latter, more decoupled topology is favourable in that it minimises the risks involved in breaking out one sub-optimally performing layer.
Conceptually speaking there would seem to be something to be gained from going with the vertical topoloy, mnimising relationships, points of contact building efficiencies. I also however understand Sinclairs concerns about the risk of a monolith.