multi-tenant vs. multi-cluster

We're considering using postgres as a way to host database services for many, many independent applications. One obvious way to do this is with schemas, roles, and proper permissions, but that still leaves open the possibility for some poorly written application to leave open transactions and affect others.

Another possible way to go is to use debian's ability to easily run multiple clusters, but while this option gives better isolation, I imaging it comes at a cost of more overhead, both in terms of memory and in terms of support. I suppose my question is, how much overhead? I'm less worried about support (that's what scripts are for) but if we're talking an extra 50MB of memory per cluster, that will start to add up.

How have you guys offered multi-tenant postgres services? Am I forgetting something?