Before iOS 4 - jailbreak software changes could do this.
For iOS 4 and later, the lock function easily sets a portrait mode for all apps that run in both portrait and landscape.
You can lock the screen rotation on the iPhone 3GS and 4, running iOS 4, to portrait only by double pressing the Home button (bringing up the task switcher), swiping right to show the media controls and pressing the left most icon (looks like a refresh button), the rotation lock. When locked you will see a padlock in the icon and a new status bar icon of the same design.
Note: this does not affect applications which only run in landscape mode, such as most games.
Edit: Video of how to do this: http://www.youtube.com/watch?v=xYkUzYXtZT0
Well since you are using Mac OSX, when you plug in your phone you can do the following to back up your pictures OUTSIDE of iTunes.
Step 1
Open up Preview with your phone plugged in. Go to File > Import Image. You will see it populate all your photos/videos. Save them in a separate folder (maybe somewhere on your desktop), then delete them all from your phone.
If you are concerned about mass deleting these files, you can just do it with SSH. This is after you've imported all your images through preview.
From Terminal on your desktop type
ssh root@your-iphones-ip-address-here
Will prompt you with a PW, chances are if you haven't changed it it's alpine or Alpine. Change directories via the following command:
cd /var/mobile/Media/DCIM
Depending on the amount of images/videos you have you will see 100APPLE/ and maybe 101APPLE/. In the directory type this to remove ALL images and movies in one command
rm -r 100APPLE/
rm -r 101APPLE/
Backup in iTunes now and you have a fresh backup with just numbers/apps/whatever else. You can then import the pictures/videos back if you want after you restore it.
Best Answer
SBRotator is a bit awful, but it works.