Support are quoting Apple's engineers as saying it is "expected behaviour" under Lion for the MBP's internal display to reactivate upon opening the lid, even if you want to use only your external display. A new version of the clamshell mode-related knowledgebase article (replacing HT3131) is due shortly confirming this.
This is despite:
- the obvious temperature-related benefits that were achievable (under Snow Leopard) by opening your MBP lid whilst continuing to output solely to, say, a 27" cinema display
- the wifi connectivity benefits of having the lid open
- the waste of energy and inefficient use of graphics processing power in needlessly running two active screens (ie if running your MBP in clamshell mode would otherwise make it too hot and/or noisy or you need it open for wifi)
- the negative longer-term impact on the battery and other components from running the MBP any hotter than it needs to be.
If you would like to have the previous behaviour restored, please lobby Apple to change how Lion handles clamshell mode. Options available to them include changing Lion's expected behaviour or adding a user-selectable option within System Preferences.
Feedback can be submitted here:
http://www.apple.com/feedback/macosx.html