ya, with the lid closed it can't dissipate heat off the top case as well so she's gonna run a little hotter then normal. Also the powerbook's energy settings are going to not be as conservative as they would when unplugged(i assume it's got ac power coming in when attached to the display as well). So the computer is gonna run "faster" therefore get warmer, and more likely kick thee old fans on. So ya, that's expected, no worries.