I've noticed my N900 getting warm when using Joikuspot but nothing alarming.

Haven't tried to use it with a car charger but I wouldn't be surprised if that didn't supply enough power for something like Joikuspot. As roundyz said, Joikuspot makes intensive use of both wifi and 3G - the two most battery draining features of this phone. When I used my Nokia E90 with a car charger, it didn't supply enough power for power-hungry tasks like video streaming over 3G. Joikuspot probably falls into the same category.