Forum OpenACS Q&A: Response to OpenACS scalability, and testing.
Some testing has been done, and the underlying software is pretty reliable. Most sites that have heavy load seem to end up caching parts of pages, and "memoizing" expensive SQL queries (basically caching the results of an expensive SQL query by only running the query at most every x seconds).
I have done some stress testing of PG and have found it to be very reliable. I haven't had problems with database corruption.
Maintaining the site, in my opinion, has more to do with changes that you need to make to stay current (add content, change HTML layout, etc.) rather than worrying about the server staying up.