S2U2 is the best option for this, it's free and has an option to deactivate all hard buttons with a slide to answer and full screen caller ID. As mentioned above you can also try TouchLockPro but, despite being smaller, I find S2U2's option list and features to be superior. Pocketshield is another option but that's commercial, I bought it a few months back and used it for a bit but eventually went back to S2U2.
Adding another vote to S2U2 (pick up from ac-s2.com).
GeSte - this also gives you the ability to lock phone on answer depending on profile (sort of) which will save those embarrassing `conference international friend into the middle of a work call` experiences that plagued me a couple of times before discovering the application.
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