If you really 'love taking video' and can do with the lower-end still-photog capabilities of the D90, you can seriously consider selling your D300 IMO.
Already for me, I don't 'love' taking video (becos I hate spending time editing) but if I get a D90 it will mean I won't need a P&S for informal family videos. So it is a big plus becos I won't have to budget for 2 cams any more!
But right now I'm keen on the D300 because it simply has better handling - dedicated controls for AF & metering, and better VF info - which I always enjoyed with my film SLRs. Plus I can use some MF lenses I still have, with metering.
Still undecided so I can understand your dilemma.