I think Nikon is loosing the market share coz they are slow to react. Canon is already out with its 18MP 550D within a year of launching 500D.Its sometimes a good idea to come out with a new model after your competitor," second mouse gets the cheese" but if you wait too long then you are bound to loose the race.
I hope D90 replacement is just around the corner.
Nikon seems to be slower and slower at responding to market challenges. At the top end, for PJ/Sports work where high ISO performance and auto-focus are king and resolution needs are not as great their offerings are, for the moment anyway, more than competitive. It remains to be seen to what extent Canon have resolved their auto-focus and high ISO noise issues with the 1D MKIV and the yet-to-be-released 1Ds MKIV in the PJ/Sports market, but it seems that they are moving along at quite a clip in their other offerings and are bound to "get it right" sooner than later. If the expected to be released soon 60D lives up the the rumored specifications it could, potentially, take a lot of market share from Nikon.
I know I would find an 18 MP APS-C sensor with good performance up to ISO 3200 or even 6400 to be very tempting indeed. Especially if Nikon were to offer it.
I believe Nikon have got to work on shortening their product development cycle. Although this especially applies to camera bodies, it is also true of their lenses, many of which are in need of being updated. What may have been sufficient in the past will leave them in the dust of the competition in the future.
There is an interesting write (with images) up of the newly released Nikon lenses on Bob Krist's blog.
I hope the rest are as good as these two seem to be.