This week I was able to do some load testing against a bunch of the EC2 instance sizes. The great news is that the instance performance we've all gotten to know around here (150/250/350 for small/large/x-large) has greatly improved with Amazon's current infrastructure and the new Wowza code. Here's the nutshell version, full details and methodology are on my blog at
m1.small: 250 Mbps
m1.medium: 500 Mbps
c1.medium: 950Mbps (full gigabit!)
m1.large: 700 Mbps
m1.xlarge: 950 Mbps
m2.xlarge: 300 Mbps
c1.xlarge: 950 Mbps
m2.2xlarge: 950 Mbps
Test was run from softlayer in Washington DC to us-east-1, which is approximately 2ms and 14 hops away.
Disclaimer: these numbers are by no means authoritative or official, but they're a rough guideline on what you can expect from those instance sizes. Your mileage may vary, void where prohibited.