I have a real rough time believing that a company's patent infringement liability only begins when they are notified by the patent owner.  It just seems a stretch, like "sorry officer, for running that stop sign - I was looking the other direction so I didn't see it".  But I suppose I could buy it depending on circumstances. 

However, I would think that if the infringing company ever publicly acknowledged existence of the patent in any form prior to being officially notified, they'd be on the hook at least from the date they acknowledged.  Here, ARM comes to mind.  They cited MMP patents in their own patent applications, thereby acknowledging existence - probably years before having been notified.

My reasoning?  It seems that avoiding liability for prior infringement would be basically "pleading ignorance" - or "how can I be liable for things I didn't know about (and would have to expend significant resources to discover)?".  That's believeable.  But once there is any acknowledgment of existence, that avenue evaporates. 

Along these lines, with even the relatively little publicity the MMP has received, IMO the existence of the MMP has been well known in the industry for year(s).  IMO, at this point, it would be pretty difficult for anyone in the industry, or in the industries of those who have bought a license (e.g., toy companies, who should know about Mattel) to "plead ignorance".  IMO, companies are on top of every move of/PR involving their competition.  And I'd bet this thought process was behind the "first mover discounts in each market segment/industry" strategy - the urgency of "getting the word out" to each, blocking each from the ability to "plead ignorance" from that day forth.

JMHO,

SGE