I've got exactly the same problem. I assume you have Maya 7.0 as thats what happens when I use that version.
I'm currently using 6.5 until I can find a solution or one of the good peeps on these boards finds a solution. So it might be best to stick with that for the time being.
I think this has the same problem and a possible fix.
I seem to have fixed it, thanks to the post by Varley. When you create your image plane, go into the hypershade and assign your image as normal. Then with the material selected (still in Hypershade) Go down to where it says 'Hardware texturing.
Change your texture resolution from Default to Highest.
You could try the other resolutions, but Highest has worked for me.
Hope this helps!!