Same boat as you, I sold my D90 some time back, got a Panasonic GF2 for fun since it was only $200 for the body. Got the Panny 14mm 2.5 and am not satisfied with the low light capability since the ISO limit on this GF2 only gets to 800-1600 and I see noticeable noise.
Waiting for the OM-D to drop in price.
My suggestions: forget the battery grip, although it looks cool I don't know how your battery situation is generally but I would wait to get the grip or just get a second battery. With the grip the size of the camera is going to go from small to medium and then your issue with size doesn't make sense then. Instead of the 12mm f/2 why not get the 20mm f1/.7 for speed? Unless you need that wide angle but then again the Panny 14mm 2.5 with the OM-D's high ISO capabilities should be fine for semi-dark situations when you must go wide and can't use your 20mm 1.7. For the price of the 12mm f2 you can get both the 14mm 2.5 and the 20mm 1.7 and still save some cash.
Also you do know the crop factor is 2x right? So that Olympus 45mm 1.8 is going to feel like ~60mm or so on your D90.
Good points all around, and I appreciate your input

I have a couple questions/comments...
Can you comment at all on the build quality of those lenses? A big thing about the OM-D that's drawing me in, is the universal praise it seems to be getting over its magnesium/metal construction and just overall "tough" feel. This is a camera that will likely take quite a beating over the years; I just can't baby it too much with it will be getting crammed into baggage, tossed around in the plane, and won't get dedicated attention when I'm grabbing it to hit the town. I like the 12mm f2, as it sounds like, from all I've gathered, another really well-built piece of kit.
I am aware it's a 2x crop, and that also kind of scares me about the 20mm. The 12mm working as a 24mm seems much better for "up close and personal" photography; a 40mm equivalent kind of scares me when I'm trying to grab pictures with my friends at a concert or indoor music festival or bar or trying to grab pictures of street graffiti, etc.
All that said, I'll definitely be taking your suggestions to heart. I need to trek out 400 miles to the nice camera store in St. Louis and get some hands on time with all of this stuff.
I may just rent a bunch of lenses when I buy my body, and see what I like best.
Probably won't change much. I doubt the new batch of Pens (rumored to be shown in September) will steal much of the thunder. Olympus is supposed to make different kinds of OM-D cameras, but nothing on the horizon.
In terms of competititon, Fuji aims too high. Still no Canon mirrorless annoucement. No sign Samsung wants to compete in that market. No rumbling about a possible NEX7 replacement. The only possible rival is the Panasonic GH3.
I suspect Olympus might bundle stuff (like the battery grip) instead of dropping prices at a later point.
Thanks! That makes me feel better. Again, I don't care if the value proposition gets better in the next 12 months; but I don't want my camera to barely be worth half of what I spent in a year.
And just fyi, the reason I want the grip is two fold - one, I have large hands. I'm 6'2" and 250lbs; I'm going to absolutely dwarf the OM-D. That said, a big DSLR is just getting unwieldy with what I'm doing. And two, I would like to have dual batteries, as I don't want to have to bother with a charger when I leave. Nevertheless, I get your point, and might play around with one before buying.
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In any case, just ordered this off Amazon:
My old Canon used CompactFlash, and so I stole a 2GB SD card from my DSiXL for the last week for my new D90. Figured this would do well for the D90; and if I get seriously going in a certain direction, I'll snag a 16GB or 32GB model, and I'll use this 8GB as a backup.