Yeah, I've been concerned about this for awhile and have written a few posts on it. I have followed Verlander on and off throughout his career, mainly using mlb.tv to watch his games. I remember when he came up he pitched a ridiculous game against Texas and looked like a scary-good power pitcher--every fastball was 96-99 and his curve was buckling knees.
This was back in 06, of course. In 07, he started out the season with pretty much the same velocity and carried it deep into the season (if I remember correctly, he ended his perfect game w/ a 98 mph fastball to JJ Hardy that Hardy hit for a FB out). But in August and September there was definitely a noticeable change in his velocity, down below 95 mph. I'd say he averaged 93 mph the last 2 months of the season and the Detroit commentators had definitely picked up on it and were speculating that it was late-season fatigue.
This season, however, he has come out and hasn't shown the same velocity. I'm not saying he is concealing an arm injury--this may just be natural wear-and-tear slowing him down. There are plentiful examples of this: teammate Jeremy Bonderman is one (people still talk about his mid to high 90's fastball, which he showed in his first 2 seasons, but really the guy throws 90-92 nowadays), Cubs ace Carlos Zambrano is another (his last few games he's actually been throwing 88-90 mph on his fastball, although he gets a lot of movement so it works for him).
The question is how much it will hurt his effectiveness long-term. To be fair to Verlander, he has an excellent changeup (+ movement and + command of the pitch), which means he can get away with a less than spectacular fastball in many games. But don't expect him to post a breakout sub-3.00 ERA/1.10 WHIP campaign with 200 K's this year. I'm going to guess a slight regression from last year to 16 wins, 3.75 ERA, 1.25 WHIP, 160 K.