My Sesnation will get a little warm from time to time, especially when streaming videos, or viewing movies over HTC Watch. It seems to happen a little more quickly when charging at the same time. There's a lot going on in a device this advanced, with a screen that large and with a battery big enough to power it all. Plus, with the unibody/metal case construction, there's not as much plastic to insulate the user as with other devices.
With that said, there is definitely a difference between "warm" and "too warm." If you have any concern that the device is too warm, please contact customer care or a sales office for troubleshooting.
I hope this helps, and I hope you enjoy your Sensation as much as I do!