Having a save game file for the original Dead Space hidden on your hard drive will unlock the original plasma cutter in Dead Space 2 right from the beginning. This has been confirmed by a member of the programming team's recent tweet.
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It's unclear how this will differ from the Dead Space 2 plasma cutter. Whether it's simply a different skin/model or whether it's more powerful.
All that's known at this point is that the save file doesn't have to be anything special. You don't need to have completed the game, so if you've since lost your save file - we wouldn't blame you, two years is a long time in video games - then you can simply load up the game and save at your earliest convenience for the bonus.
Careful though, you may just find yourself playing through the entire game again as a result.