Simple: Use the free Handbrake application
http://handbrake.fr/downloads.php
Once you move the movie from your iPhone to your computer, find the file. Launch Handbrake then open the file with Handbrake (you need to navigate to the file within the Handbrake > open file dialogue.
If you are good, Handbrake will open the file IN LANDSCAPE mode
Now, choose the Output settings (MP4 file works great) and hit the START icon. The corrected, landscape oriented, movie will appear on your desktop. Open with QuickTime to confirm (or QuickView) then delete the old file.
You can now sync the corrected video back to iPhone with iPhoto.