First, make sure that's what you want to do! Read this most excellent article about why the philosophy of DVCSes might be better than that of Subversion:
I don't know whether you'll agree with Joel or not, but you should consider that there are more modern approaches to the subject than Subversion's "CVS without the suck" model.
Is the problem the decentralized nature or the differences with how it works?
If it's the former, this page has some discussion of using Mercurial with a "single central repository."
If it's the latter, I hope you'll all give it some more time. The reasons you migrated in the first place are probably still valid.
Unfortunately, I can't find any discussion at all about people migrating from Mercurial to Subversion. You might need to hack together the equivalent of a Subversion dumpfile (like the output of cvs2svn).
You should probably discuss this with firstname.lastname@example.org:
That's probably the best place to troll around for advice (or maybe even a script) for doing what you want.