I don't think it makes sense to spend time on SVN support.
Any .NET shop who is stuck on SVN is NOT going to outsource their deployment/hosting to AppHarbor. It's better to serve the people on the cutting edge first.
Wrong. The reason why Git/Mercurial users are more likely to use AppHarbor is that AppHarbor is probably more likely used by open source projects. Enterprises, where SVN is actually the preferable system, are less likely to host on AppHarbor (or EC2 or Azure).
The issue isn't cutting edge of the source code control system (which frankly makes no sense if you actually understand Git and SVN), but business model.
The issue isn't cutting edge of the source code control system (which frankly makes no sense if you actually understand Git and SVN), but business model.