I really like your rankings a lot. Most people post there rankings to get some kind of feedback - constructive criticism if you will - so here goes.
QBs - really good looking ranks, although I agree with the others that MCNabb really needs to be in the top tier with the other 2.
In Teir 5 - there is no possible way you can convince me Carr and Rothlisberger should be ranked that high - just drop them completely from your rankings.
RBs - McGahee is too high. At a minimum drop himto the 2nd tier - personally I would have him in the third tier. Id switch Rudi and Tiki. Portis that low is really questionable - Id urge you to reconcider. I have Portis firmly in that 3rd tier. The rest of the RB rankings look really solid.
WRs - I love that you have caught on to Harrisons decline - nicely done. Wayne could easily out produce Harrison - it wouldnt suprise me in the least. I assume you have dropped TO for health or other reasons that could make him lose time - otherwise he should be a top 3 pick all day. I really like your tier 3 - AJohnson shouldnt be a top 10 WR as everyone and their brother are predicting, so 13th as you have him looks really solid. Tier 4 - Porter is overhyped - you have him ranked too high. CChambers is ranked way too low as MIA will look for him much more than in the past - very very good WR. I would go so far as to say switch Porter and Chambers on your list. Jimmy Smith really needs to be in tier 4.
Love your TE rankings, although I would switch it up a bit by singling out Gonzo in a tier by himself and then putting only Gates and Witten in the 2nd tier. Teir 3 would be Shockey, Crumpler, and possibly Heap.
Overall - this was really nicely done - you obviously put a lot of thought into these tiers. Good luck.