Why is the Recovery HD mounted when running Software Update?
Software Update checks and updates both your running Lion and your recovery partition, so that in the event of a significant security issue booting to the recovery partition won't instantly open you up to attack.  (Compare booting unpatched Windows XP from DVD or a vendor recovery partition for recovery purposes; it has severe security issues fixed by service packs.  I'm not picking on Windows here; XP's quite old at this point, and was the biggest target out there for years, and security is a rapidly moving target.)  Given the recent growth in Mac malware, this is a good thing.