It's not all about speed anymore, but it's still a very big part of the game. Acceleration has taken over in a few areas e.g. it's more important for a DE/OLB to have high acceleration than high speed.
But play any player with a high speed rating and high agility rating (that allows you to do 90 degree, 1 yard turns at full speed) and you'll dominate at almost any position.
For receivers speed is the most important rating for streaks, but DB's are given high speed/agility ratings as well so you don't dominate too much. Also for receivers that does a lot of route running - yep, you've guessed it - the route running rating is extremely important, as the game gives a speed burst after cuts to players with high route running abilities. Players like Gates, Wayne will get open almost every time except when covered by one of the few 99 man coverage rated DB's in the game.
For linemen and linebackers my experience was that acceleration and agility was very important when rushing the passer, unless of course you get sucked into a interaction animation with a blocker, then block shedding is the key rating.
Speed is still a very important rating, but not as big a factor as earlier. EA's tried to fix that with Madden 11, but the current speed bursts, block shedding, WR release issues etc. has made Madden gameplay more unrealistic and inconsistent IMO. To sum it up, Madden 11 is the edition of Madden that I have played the least since I started playing Madden.