Cortex-M3 is quite new, so vendors are pricing their products very agressively to gain market share.
With Cortex-M3 parts available at around a dollar, the future does look questionable for the high-end 8051s - assuming that this pricing is sustainable...

It certainly means that there is really no point in struggling with and 8051-based design when a Cortex would be a breeze (ie, when you're struggling with things like code space, data space, or any other specific limitation due to the 8051 architecture).

I'm sure 8-bitters will continue to have a place at the really low end.

There is still a cost in acquiring tools, and the learning curve for both the tools and the architecture - so that will continue to make 8051-based solutions viable for many developers.