There's been a bunch of chatter on the gnucash mailing list about a SQL backend lately. Its been dabbled with many a time in the past, but never fully implemented. Someone pointed out that there are some column definitions coded in the source. That was cool to find. I responded to that with a comment about the XBRL specs and how I've been trying to implement them into the PBooks data model.
Wouldn't it be cool if you could have a desktop application like GnuCash access the same database as a web-based bookkeeping system like PBooks? I think so!
UPDATE: Derek on the mailing list clarified that the code referenced above was used with GDA (Gnome Data Access), and the current GnuCash application uses the QOF (and ^).
UPDATE Feb 7, 2008: I took another crack at this with mixed results:
