I'm using Arc at my work as a webapp. Really not a web site, just an app with a web frontend. I first used Arc0 (yep !) Switching to Arc1 was breaking a few minor things (the thread function changed). Switching to Arc2 did not cause any problem.
This is a single Arc process. As I needed it to be secured (through SSL), I use an Apache frontend. Requests on the SSL port are redirected on the 8080 port, which is Arc's port. Accessing directly this port (that is, not through SSL), is forbidden (see defsop macro).
I'ms using Ubuntu's 7.10 version of Mzscheme (360). I don't have any persistence problem (this is only a frontend to the underlying filesystem, and logins/passwords are dealt by Arc's web server). I don't have any probleme with access time, scaling and such things as I have only very few users and as it is a local application.
Adding / removing / adapting features in this app is a real pleasure. I never felt so productive in developing a webapp. Now, of course, this is a small one, I don't know how it feels to develop, say, a Hacker News forum...