Honestly, PC people...when is the last time you upgraded your CPU? I am asking because I've built more than 20 pc's in my lifetime over the years (I don't do it anymore because i'm busy) and have never upgraded the CPU or graphics card. By the time I need to upgrade, new chipsets are out so I have to upgrade the motherboard & gpu as well as hard drives.
To say that PC's have upgradability, is true, but to a certain extent. If you're one of those super technical people who gets off upgrading CPUs, etc, then go for a PC.
You will definitely save some money when you get a PC (especially a laptop) but that $400-500 that you save will cost you later. You might get a few extra USB ports, a bigger drive, a blu ray drive, etc., but when it comes down to essentially using a machine productively, you can't really beat a Mac.
To top it, if you really do like to be "cutting edge" (I loathe this term) then just resell every 2 years and get the newest mac. No need to worry about upgrades etc. Better yet, if you pop in more Ram, buyers will eat it up. Increasing your resale value.
One thing to note is that if you're just going to be using it as a Facebook machine, please don't get the most expensive MBP, making it into a $2,500 Facebook machine is really irresponsible. Buy yourself books instead! If you're definitely going to be doing many things, especially creative work (ie, music photo editing, video editing, etc.) and want to become at least average at it, then by all means upgrade the speed a bit.
In this economy, it doesn't make sense when you buy a $2,500 machine just to browse the web and write papers. You look like an extreme troll with that illuminated Apple logo sitting at a Starbucks checking your Facebook page and instant messaging your "bff".