No warranty of any kind implied or given and no liability accepted for any loss, damage or injury, no matter how incurred.
I've been researching this for a while and the consensus appears to be that, on the M3 at least, it is not an if it goes but a when.
Here's some handy links, the PDF is the best I have seen for info on how to fit one of the reinforcing kits available.
The URL for buying the kit (they deserve to make some money for the detail in the PDF alone)
E46 Rear Chassis/Subframe Reinforcement Kit - Turner Motorsport
And the gallery.............
BMW E46 Subframe Failure
I've noticed a couple of kits out there, one where some of the cracks were welded up outside the area of the reinforcing plates, this one looks big enough to cover all but don't take my word for it as not done the job with any yet.
The easiest way to fix this appears to be to have a car with full history, standard suspension and less than 10 years old and approach the dealers for assistance.