No, the 300D's autofocus is not bad. I have looked into this, as I was wondering why my 75-300mm IS USM
seemed to focus a bit slowly - it's because there's so little light at 5.6 aperature. I tried out the 2.8 L lens, and the focus was nearly instant. The lens has a LOT to do with it.
Also, the theory you mentioned of the 300D's images being slightly soft is also not true. In fact, it's been posted on this forum several times that if you set the 300D's sharpness to -2, it's equivelant to the 10D's -0- setting. I've taken a few shots in large JPG mode, and the sharpness is incredible. Unfortunately you can't really keep that much detail in a shot that has to be 1024 pixels wide, so you lose a LOT of detail when you resize for upload. But I've got some 3072 x 2048 images of the 727 that were shot from in front of the nose, and you can virtually count the rivets on the TAIL. After moving up from a 4 megapixel point-and-shoot, the sharpness of the 300D is amazing to me.
Check the sharpness of the 300D (Canon Digital Rebel) in these photos:
As Tim mentions, sometimes the images out of the 300D are so good, there is no need for post-processing!
Hope this helps!