I made a long post about this on another message board sometime ago. To sum it up, it's stupid. In fact, I think having a top 10 pick kills your team in the long run, compared to the pre-salary cap era when having a top 10 pick was great for your team.
1) Drafting an offensive player, you probably don't know what you're getting for years unless it's a RB. Having three years worth of money tied up into a high priced player who doesn't perform was a waste of money. A good example of this is Joey Harrington. It's year three and Detroit still isn't sure what they have on their hands. A lot of money is invested in this #3 pick, and if he isn't any good, that hurts Detroit for years. His base salary escalates to a shade under $5 mil next year too.
2) I think there's better value in trading your draft pick down for more picks/players. It's better for the salary cap, and being able to stock up on late 1st rounders to 3rd round players gives you immediate depth which is good for special teams among other things.
3) There seems to be less of a learning curve for defensive players. You tend to see what you're getting right away. If you're going to draft players high, go with defense. One of the reasons I have Chicago better than Detroit over the long haul. Apparently Tommie Harris has been great for them so far and I've heard Tank Johnson is showing promise too (they did a good job against the Philly interior last Sunday).
4) Recent history shows going with an offensive lineman is your best bet if you draft an offensive player.