EBS is bounded at 1TB / volume. Then you have to stripe.
With EC2, I can have a server half the price of Rackspace's cheapest instance, but with theoretically unlimited storage space. I could use that e.g. for backing up server data, or as an NFS share.
The one major limitation of Rackspace's service is disk space. It's severely limited, and there's not much you can do about it.
You get /dev/sd[b-p][1-15] as mountable EBS volumes. Thats (15 * 15) * 1TB, or about 225TB. You may even be able to go higher than "p", but I haven't tried. I'm also not sure, you may even be able to attach somewhere other than /dev/sd* .
Either way, after that, your EBS bill is $22,500/mo, just for storage, so you might be able to figure something else out for that cost.
But you CAN attach that to a $14/mo EC2 instance, which was my original point.
Edited to add:
Note that the default limit is 20 EBS volumes per EC2 account. You can request an increase from Amazon if you need more. http://groups.google.com/group/ec2ubuntu/web/raid-0-on-ec2-e...
Still, 20TB is "practically unlimited", for most applications.
Also, in response to your question: I can confirm that EC2 complains if you try going above sdp, or anywhere else.