For everyone asking why you would ever need 8gb of RAM, if you make music on your mac, you would know.
There are many applications that require tons of RAM, from my personal experience, I can talk about music. These days you can plug your guitar into your mac via an interface and pull up an amp emulator and effects pedals as software and record some pretty crazy and unique sounds, without the need to spend thousands on expensive amps and pedals.. you can use a USB keyboard and make it sound like a $400,000 Yamaha Grand piano in a huge hall. You can make your electric drum set sound like Ringo Starr's Ludwig set from 1965...
but all of these software plugins load directly into your RAM! So the more software plugins/emulators of these kinds that you use, the more RAM you need.
I currently use a 13" MBP with 2gb of ram, and I'm NOT upgrading to 4gb, instead I am waiting til the price of 8gb goes down so I can do that. Because I NEED 8gb of memory to make my songs sound better.
Like I said, there are other applications heavily dependent on memory, such as Final Cut Studio, etc. but my personal experience is with music making, specifically Logic Studio 8.
I could have gotten a Mac Pro with 16gb of RAM, but I couldn't afford that and I needed something really portable that I could just plug my guitar and small 49 key keyboard into and make quality music on the go.