Update 3 Jan 2012:
I just spoke with both companies, and had positive responses from both.
Kahr didn't give me much information, but thought with the side plate missing the gun had performed correctly (as several have said) so I will be sending it back to them at my expense for a gunsmith to examine. They didn't ask what kind of ammunition or powder or anything, but gave me a return number and asked that I put a note to the gunsmith in there. Being remanufactured ammo it will most likely void the warranty, but I'll be putting that on the note, so we will see.
Ultramax customer service immediately transferred me to a man named Larry, who was very helpful. He asked for lots of details and for the lot numbers. The ammo I had was from either 1991, and he explained that it was most likely a problem with the roll sizes, which would change where the pressure went. He said in the company's early days this was not monitored like it is currently, and several employees were let go because of it.
I obviously can't speak to the authenticity of that information, but their following actions make me think this is a pretty decent company.
Without hesitation he offered the following:
1) Paid postage for a return of all my Ultramax ammo, including 2 boxes I had in another caliber
2) Money back for all of that ammo or new, replacement ammo (specifically mentioned all would be FMJ)
3) Covering my postage of my gun back to Kahr, and any repairs or replacement costs of the gun if the gunsmiths think its too far gone.
While I don't claim to be an expert in reading people, Larry seemed both concerned and ready to help. I'll be taking the replacement ammo option and giving it another try as I bought into the company line of "a lot has changed in 20 years".
I'll post again when I hear more information from Kahr.