Using micro instances I've seen it go up to 99% during CPU intensive work (ex: app build). The hang ups waiting for it continue made me decide to switch the build server to an m1.small instance instead. It's idle the vast majority of the time but the extra $$ for it is totally worth it when you're running a build.

The steal % is usually zero on the m1.small instance. I just tried maxing out the cpu and watching "top" this is as high as it got:

    Cpu(s):  5.3%us, 39.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si, 55.7%st

To max out the cpu I ran the following in a separate ssh terminal while watching "top". An m1.small only has a single v-cpu so only a single running copy should be necessary.

    while :; do date > /dev/null ; done