Sounds like you were in manual and didnt adjust aperture and/or ISO accordingly. Slower shutter speeds needed for prop blur let in more light, so you have to subsequently reduce the light for a correct exposure by letting less in (aperture to f8,f11,f16,f22) or reducing light sensitivity (ISO 50,100,200 instead of 800,1600,3200). The three are interrelated, so if you halve the shutter from 1/125 to 1/60 you will need to go from f5.6 to f8 or from ISO 400 to 200 to maintain roughly the same exposure - although both may introduce other effects you may or may not want.
It is possible in very bright light - especially with point and shoots, to find the light so bright that the camera can't achieve a speed slow enough without over-exposing. I had that at Silverstone once with my S1IS when trying for wheel blur. For a DSLR this situation is less likely because you normally have access to much smaller apertures, but the solution is to use a ND filter or a lens which has more f stops.