Just how secure is a pattern lock? I recently had a phone stolen. It's been replaced, I've changed my passwords, the phone company has shut down connectivity for the stolen one... I think I'm pretty much as safe as I can be.

However, it did get me wondering. How secure is the pattern lock? My phone had a pattern lock, so I assume that means that the casual user can't pick it up and do anything.

But if one had all the time in the world and some technical know-how, could they circumvent it?

Or is the phone only useable if the person wipes it and starts over?

Note 1: I know the data on the SD card is a separate issue, I'm just wondering about the phone and it's internally stored data.

Note 2: I've seen other questions about recovery of the password via one's Google account, but the person who has taken my phone (presumeably) doesn't have the ability to reset my pattern that way, so I believe this question is a separate issue.
You don't even need much technical know how to crack most touch screen phones. There was a white-paper written recently about the security of touch screen lock mechanisms (and pattern locks in particular) and breaking into these devices using purely the greasy smudges on the screen to get the unlock code. 


  8 Conclusion In this paper we explored smudge attacks using residual oils on touch screen devices. We investigated the feasibility
  of capturing such smudges, focusing on its effect on the password
  pattern of Android smartphones. Using photographs taken under a
  variety of lighting and camera positions, we showed that in many
  situations full or partial pattern recovery is possible, even with
  smudge “noise” from simulated application usage or distortion caused
  by incidental clothing contact. We have also outlined how an attacker
  could use the information gained from a smudge attack to improve the
  likelihood of guessing a user’s patterns.
  ...


Smudge Attacks on Smartphone Touch Screens (PDF)

The rest of the paper is well worth a read too.