10.7.3 - Screensaver turns on while I'm in the middle of typing/mouse activity
According to Apple Support:


  If you use a screen saver, it starts automatically after your computer
  is inactive for a time. You can also choose a shortcut so your
  screen saver starts when you move the pointer to a corner of the
  screen.
  
  
  Choose Apple menu > System Preferences, click Desktop &amp; Screen Saver, and then click Screen Saver.
  Open the Screen Saver pane of Desktop &amp; Screen Saver preferences
  Click Hot Corners. Choose Start Screen Saver or Disable Screen Saver from the pop-up menu for a corner you want to use; and
  then click OK. To use the Command (⌘), Control
  (^), Option (⌥), or Shift (⇧) key
  together with a hot corner, press the key while the pop-up menu is
  open. For example, if you press the Shift key, the Start Screen Saver
  command changes to ⇧-Start Screen Saver.
  
  
  To deactivate the screen saver and return to the desktop, press any
  key or move the mouse.


Additional screensaver configuration options for Lion are listed in this Apple Support KB.

This issue could definitely be a bug, but you could also check your screensaver configuration to determine whether you've turned on hot corners - it's possible that your mouse pointer may have triggered the screensaver. You can also choose to disable screensavers completely to avoid this issue.