An update on the mono memory issue
The workaround I tested a few weeks ago did not really solve my problem – actually it made it worse. The autorestart caused mono to hang and not restart at all, so my site stopped responding every 6th hour. I quickly had to disable this. Still I had the memory consumption problems. From various sources I was told that the memory issues would be fixed in the recently released mono 2.4. Also I found a bug report regarding AutoRestart, which also should have been fixed in 2.4. So I decided to give Mono 2.4 a try.
The problem was that there were no packages for Ubuntu 8.10, so I had to download the sources and build my own deb packages. I found this great blog post that describes the process of building and installing Mono 2.4.
So far it seems to be working, but it is too early to say if the memory consumption issues are resolved.