As I say Toyota's have the same problem and they won't fix it either. It's just the type of metal apparently.
By the way, the comment about wheels being outside warrenty. Other manufacturers guarantee their wheels.
I had most of the wheels replaced 3 times on my Celica (at least 3 of the wheels were changed on an annual basis) due to corrosion. I know a couple of people who had MX5's with a similar issue. Both manfufacturers replace wheels under warrenty with no issues.
I have had a full set of alloys under warrenty on my LCR so they do cover wheels.