When you are preparing for a 2009 fantasy football draft, you want to use whatever information is available to help you project (or is it "guess"?) how players will perform in 2009. Typically rankings rely heavily on players' numbers from the previous year, but sometimes take into account how a player has performed in multiple previous seasons, too.
I think when assessing fantasy potential, we should include playoff games, too. Playoff games provide more data to sift, more material to assess. Sure, playoff numbers don't count for your fantasy league, but no 2008 numbers will count for your 2009 fantasy league. You're trying to rank players based on how you think they will perform--why not use any available data that could help you to project accurately?
When I rank players for 2009, I will look at all the games the player played in that counted. I'll look at the per game averages and the number of quality games, and rank players accordingly.
Now a key point here is that for fantasy analysis purposes, playoff games be regarded as equal to regular season games. If a player significantly overperformed or underperformed his regular season numbers in the playoffs, you don't significantly mark him up or down for that alone. The playoff games go into the total pool of assessable games, and you then assess accordingly.
Including playoff games will not have a giant impact on your rankings (for most players it adds one to two games to his total), but it might allow you some keener insights. Larry Fitzgerald was already going to be one of the top three WRs selected; being the indisputable best player during the playoffs simply makes him the consensus #1 WR. Santonio Holmes was a 2008 fantasy disappointment, yet he made big plays in the playoffs (scoring a TD in all three games, winning Super Bowl MVP). While Holmes is a potential #1 WR, even his playoff numbers suggest he's an inconsistent big play WR (his reception total in the playoffs: 2-2-9), great when he does make plays, frequently a nonfactor. So including playoff numbers doesn't provide you with a big secret answer for fantasy success--it just gives you a little more data to help you make decisions.