Simple...

(1) Never buy CCD based dSLR if possible... They has relatively very high electronic static (try only turn on when you need to use), also waste power and contribute to noise. Older camera are worst. I used to have unsolveable nitemare with D70...@#$%@#$%.... Trust me, that camera issue cannot be solved... 

(2) Dun change lense on windy and dusty place. It is why I carry 2 camera

(3) When change lenses, your camera should be facing down.

So far here is my record;

D70 -> everyday kena even no change lense and just serviced by Nikon.    And there is only one solution, sell it off... 
EOS 10D -> kena dust once at Bali, Indonesia because change lense near beach
EOS 30D -> kena dust once... forgot
EOS 40D -> Not Yet... But expected to kena at least once this year.

but my Canon CMOS sensor dust very simple, at home blow blow settle... 

try do ur test shot at f20 and above.

more or less, whether ur lens or ur sensor, most of the time some tiny dust will be there.

i blow my sensor and mirror, but one time i blow it with a wrong angle, the dust fly inside the penta-mirror and stay there. Nikon says need RM200+ to remove it, even still under warranty..

meaning you changed it while not facing down? bad move.

I dunno about other cams but I read from the manual that the 40D has these sticky things at the bottom of the body to attract the dust there so it doesnt fly everywhere once inside the body. It also has an anti static layer on the sensor.