Why is the Recovery HD mounted when running Software Update? I have installed HardwareGrowler, which displays a notification every time something is being mounted/ejected, connected/disconnected, etc.

It tells me that at the start of when Software Update the Recovery HD is mounted and after something like 5 seconds it is ejected.

Is this because it needs to check the version of your Recovery HD, or is it used for something else?
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.