Klaus,
Merom is definitely a new architecture. Although it may be heavily derived from Pentium M (Banias, Dothan, Yonah), but there are substantial changes. Just the headline ones - AMD64 support and 4 issue ALU make up basically a new core, rather than just a revision of the existing one.
My guess is that Intel will be conservative with other changes, since Merom is a life and death release for Intel. A failure of Merom equals failure for Intel as a whole. There is nothing left to bail Intel out. Getting substantial gains from Netburst core at reasonable performance / Watt ha been increasingly difficult.
Server version of Yonah (Sossoman?) is at best a niche product (Microsoft calls 32 bit in servers "Legacy" now) and Itanium continues its dollar short, day late routine.
Yonah looks like a great core, and alone could keep Intel profitable for another 18 months, but without Merom, there is a steep cliff for Intel beyond that.
Frankly, I am surprised that Intel is even going from 3 to 4 issue. It may turn out to be a brilliant choice if everything falls in the right place, but, IMO, it was an unnecessary risk, considering the consequences of major failure of this core for Intel.
Joe