I think this really quite simple. It's about upgrade paths and staying on the trend curve.
I suspect Nikon is just a bit ahead of the curve on this one. I am sure that Canon will also do the same now the SDHC standard has been agreed and capacity/price ratio, the last real advantage CF cards had over SD is no longer an issue.
Out of interest I checked the Canon EOS-1Ds Mark II the only DSLR to my knowledge with both CF and SD slots. The SanDisk Extreme III 1GB SD is faster than the SanDisk Extreme IV 2GB CF Card (There was no 1GB version to compare, I am aware that there is a marginal drop off in write speed as the capacity gets bigger)*
Apple also took the decision to introduce an express card slot on the new Macbooks which could only hold an SD card reader and not a CF one. This is inline with the Windows machines from Dell and HP who are shipping SD card slots on laptops. The writing is on the wall for CF imo.
Specifically in relation to the Nikon lineup this move makes sense imo. The D50 uses SD cards and so does it's bigger brother the D80. This makes the upgrade from point and shoot > basic DSLR (D50) > Serious amateur (D80) a seamless one with regard to storage. This gives a point and shoot upgrader one more reason to choose a Nikon product.
Those people with a D70/D70s are more likely to head towards a D200 rather than the D80 in any case. At this price level, I agree that the cost of purchasing storage cards is a moot point.
My own 2 cents worth says that Nikon will not launch a D90 but rather a D40 to replace the D50 and clearly differentiate it from the D80.
* Witite speeds from
http://robgalbraith.com/bins/multi_page.asp?cid=6007-7344 for RAW images.
CF cards
SanDisk Extreme IV 2GB 8.010MB/sec
SanDisk Extreme III 1GB 7.878MB/sec
SD Cards
SanDisk Extreme III 1GB 9.023MB/sec
Health warning. I am a Nikon user who switched this year from a Canon system.