I remember this was discussed a few months ago. But I think people were saying the stains were blue or purple instead of brown. Try doing a search for that instead.
I'm on Pentasa and my brother is on Asacol, and we both stain the toilet, so to speak, with urine. I mentioned this to my GI, who is very experienced treating IBD, and he said he'd never heard of that. I did a urinalysis (and recently another one) and they came back completely normal. So we concluded it's a harmless side effect of Pentasa/Asacol. I've yet to see a real scientific explanation of what's happening, but I'm not too worried about it.