Simple! It’s a shame there is no preference setting for it, but this isn’t particularly difficult!
“Here’s what to do – first you go into Terminal. If you don’t know how, just invoke Spotlight and search for terminal and launch it from there.
In Terminal, write (or copy/paste) the following line:
defaults write com.apple.QuickTimePlayerX MGPlayMovieOnOpen 1
From: How To: QuickTime X Autoplay Movies On Open In Snow Leopard — TechPatio
As my previous post, where I detailed some steps I took which seemed to help minimise the issue with my face hitting speaker (in my instance), is one of my most read posts, I thought it best to comment on iOS 4.1!
This recent update from Apple fixes this particular problem with the proximity sensor for good. I’ve not had a single issue at all with it since installing the update just over a week ago.
Updating is really easy too. It takes a while, but its worth it! Simply plug your iPhone into iTunes like you do, and you should be prompted for the update. If not, click your phone on the left hand side in iTunes and click “Check for Updates”. The download may take a bit of time, but you can use your phone while the download is happening.
I hope that helps!