wunhangios is also right. You can VERY EASILY rotate the video using iMovie on the iMac. Not sure if it can be done on the iPhone though. I played around with iMovie on my iPhone and didn't see where it was possible. The iPhone version, while pretty cool, is still very limited compared to the Mac version.
If you have a PC though, you'll have to do what I mentioned earlier. Unfortunately, Apple updated Quicktime Pro a few months ago and my old QTP license is no longer compatible. ... Thus it makes it difficult for me to explain how it could be done on a PC with Quicktime Pro. I'm sure that if you searched Google for "How to rotate a video using Quicktime Pro", there will likely be a few bazillion hits. It will probably be more of a challenge sorting through the chaff to find a recent source for the latest How-To's.
Basically, it was relatively simple to do, when it worked. Quicktime Pro was an incredibly simple program. I was a bit underwhelmed when I purchased my license, but it did what I needed it to do that I couldn't do with any of the other sources available to me at the time. But if I remember correctly, you'd just open your video, and go into the properties tab. There you would see the separate audio track and video track. You would click to highlight the video track, and I think you could right-click on it to get to an additional properties menu where you could then be able to rotate the actual video. Once you did that, you would need to actually Export the video to a new file in order to save the changes and create the correctly oriented video. If you simply chose Save.. from the Quicktime Pro File Menu, it wouldn't actually save the video how you would expect. It was kind of a pain in the rear to learn how to use, but I remember once I figured it out, it was pretty easy to do with other videos. I did discover that certain .mov videos would not work or save properly with Quicktime Pro, however it was all video that was shot with a different camera as the source and not my iPhone.
Again, let us know how it works out for you!!
All the best!