I wouldn't be too worried about Nelson's 40 time. Sub-4.6 for his size, style, and smooth route running is perfectly fine. Remember this guy is a possession receiver with an athletic body. He caught 122 passes in his senior year for something liek 1,600 yards. I think Nelson may be the most productive receiver in his rookie year in the NFL. Of course, that is a complete estimation confisidering I don't even know where he will end up. I'm very impressed with Nelson.
Right now it looks like Jordy Nelson, Early Doucet, and Malcolm Kelly are at the top of my receiver list. Followed relatively closely by Limas Sweed and Andre Caldwell (too bad he is a Gator). I am not too high on DeSean Jackson. Jackson seems like a Ted Ginn Jr. with less football IQ and worse hands.
I'd also like to add that Manningham is going to be a good WR in the NFL IMO. He just knows how to beat his man. He may not run a fast forty, but I bet you he burns some NFL corners in his rookie year.