Why is iPhoto changing the titles of my photos after uploading the files to Flickr?
I don't think this can be turned off. Another effect this has is that if you delete a photo in iPhoto, it also gets removed from Flickr.

To avoid this, i use Flickr's free Uploadr rather than the built in tool. This lets you select the photos, give custom names, etc. It's easy to use, and if you make changes to the photos in iPhoto, there will be no effect on the images on Flickr.