I'm in the same boat
I have recently been looking into this same question as I recently got an Apple TV and have been busy encoding my DVD collection, I'm running out of space on my 500Gb external drive quickly. I have a macbook pro and am considering getting an iMac within the next year. Basically that limits the options to Firewire or USB systems. The fancier, speedier RAID systems tend to be eSATA and depend on a PCI RAID card to actually do the work.
A couple options that I have looked at are Lacie Biggest Quadra (lacie.com) and Drobo (drobo.com). I have not used either of these products, I'm still in the research phase. Drobo is interesting because it is *not* RAID as we know it, it is something "smarter" according to them. It seems to have some advantages over RAID systems but also some disadvantages, for one thing it is rather slow. However, if you just want an external enclosure to serve up iTunes files (that's all I need now, I'm not doing any HD video editing) then it is certainly fast enough. I'm certainly going to wait a bit and keep an eye on reviews as it is a relatively new product from a new company. Lacie is of course much more established and well known. I really like the look of G-technology drives but it seems that their 4 bay RAID enclosures are eSATA only and require the PCIe card to do RAID 5 (my understanding is that some PCMCIA or ExpressCard eSATA adapters will do RAID 0 or 1 but haven't seen one that will do 5)
I'm definitely interested in other options, if anyone knows of any other good, simple solutions for a media RAID please add to the discussion. I'm looking for something that will do RAID 5 or equivalent and that is easy to expand in the future and not terribly expensive up front.
Edit: I just discovered there are ExpressCard eSATA adapters that will handle RAID 5... too bad my aging MBP doesn't have expressCard.