I've just submitted a patch that adds CTI to ActiveRecord. It's not quite done yet - some stuff is untested, and associations (probably) don't work with CTI classes. But you can read and write CTI records with no special syntax (it looks just like STI).
I've posted more details on my company's blog here: http://pdcm.codeweavers.net/2006/11/7/class-table-inheritance-in-activerecord
And you can download the patch from the Rails trac: http://dev.rubyonrails.org/ticket/6566
The patch is quite large - it was a lot trickier to make it work than I thought at first because ActiveRecord has a few assumptions that only hold under STI. I've avoided nasty hacks but it still took some voodoo meta-programming to preserve the existing functionality.
Class-table inheritance is one of the big feature we want at my company before we start any big Rails apps. If anyone would like to use it in their own projects don't wait to let me know I'm hoping I can finish it off in a week or two. It's hard to gauge the demand for CTI because not many people have attempted it so far.