This February 2003 Linux Kernel list thread on version control had some interesting info. From that, it seems likely that Arch is still, at least currently, noticeably inferior to BitKeeper.
E.g., Olivier Galibert said, "Hell, arch is still at the update-before-commit level. I'd have hoped PRCS would have cured that particular sickness in SCM design ages ago." which sounds interesting, although I don't know what that means exactly. (But Tom Lord thinks it mostly meaningless drivel, and explains why.) And Linus Torvalds mentioned, "Give it up. BitKeeper is simply superior to CVS/SVN, and will stay that way indefinitely since most people don't seem to even understand _why_ it is superior."
It is not clear to me whether Arch's (assumed) inferiority to BitKeeper is fundamental, or Simply a Matter of Programming. Lacking any additional information, I would tentatively conlude the latter - Arch can catch up (eventually even surpass?) BitKeeper - but that doing so is probably hard.
Even so, that doesn't matter to OpenACS, as we've been getting along ok with CVS, which is much, much more inferior to BitKeeper than Arch could be! So if Arch is judged to be, useful, currently a substantial improvement over CVS, well-designed, likely to undergo regular further improvement and maintainence, and a better choice for OpenACS than other competing Open Source revision control tools, then OpenACS should use it.
Note that above I said competing Open Source tools. What about closed source tools, like BitKeeper? The Linux kernel project uses BitKeeper. The Linux kernel is an extreme case of needing the best distributed version control system you can get, and needing it right away. Linus obviously decided that this need was so compelling and immediate that it overroad any concerns over BitKeeper's non-open-source license. OpenACS is not in that situation; we have no such extreme and overriding needs. Therefore it should be easy and best to simply side-step all the huge acrimonious licensing flamewars that the kernel folks have gone through, by simply going with the best Open Source tool, and looking at closed-source tools primarily only in order to better understand and evaluate the open source ones.
Perforce? It's closed source of course, and I've never heard any arguments on why it's better than BitKeeper. My guess is it isn't, at all. If we were going to go with a closed source tool (bad idea), I think it'd be hard to come up with any good reasons to use anything other than what the Linux kernel folks already went with - BitKeeper.
Tom Lord (original author of Arch) made some fairly insightful criticisms of SubVersion back in February 2003. Basically, he thinks the SubVersion project is fundamentlly a failure, and is interested in understanding why and how.
Roberto, I would be very interested in seeing your stuff on OpenACS switching to Gnu Arch!