I think on a scale of "powerful" CPU's it sums it up best to say the Roku doesn't have a CPU at all compared to most modern devices. Someone mentioned a microwave and that may actually be close. Roku can barely run the channels it has sometimes and I think putting programs on it would cause all sorts of unforeseen problems.
If I were going to hack something I'd get a Logitech Revue. Hardware versus cost puts it as the cheapest device out there and it's the cheapest to run Honeycomb Android as far as I know also. It's not the easiest to get root on however.
Also, don't let the fact that the Roku has linux buried deep inside of it somewhere mislead you. The box is actually using a specific software shell and gets updates from, and only from, Roku. It's not going to be easy to get to root on this box either. Some tried, on the Roku1 but I don't know if anyone succeeded. Getting it to accept software is going to be a huge stumbling block. Now if you are content to write "channels" and leaving the box as is, a media player, that might be different.
Nun sacciu, nun vidi, nun ceru e si ceru durmiv.
I know nothing, I see nothing, I wasn't there,
and if I was there, I was asleep.