I'm trying to wrap my head around some of these issues also. I'm working on two projects now that both use RDBMS backends. One of them seems like a natural for using an OODB. It's a DB of network devices (routers, switches, APs, etc) and I've built my own simple ORM for it (could have used SQLobject, I know!). In many places, I already act like I'm using an OODB. E.g. instead of using crafted SQL to pull specific devices out of the DB, I loop over them all ('SELECT * from devices'), wrap each row up as an object instance, then check attributes on them to find matches (device.Model == 'Router M10'). I'll be examining this one to see how well something like durus could replace MySQL. I imagine it would be quite easy.
OTOH, the second DB is much larger and I think of as containing dumb data. A few well crafted indexes and queries are used to dump out search results as quickly as possible. There's nothing OO about it.