First, this gives Linq to Sql a better opportunity to thrive. Linq to Sql is the "simpler" OR/M that's looking good enough for the vast majority of cases, while ADO.NET EF is far more complex -- and yet most gurus only wanted to talk about the EF.
Next, since ADO.NET EF is so complicated, it absolutely must have a great designer ship concurrently, which was not the plan. MS has apparently accepted this feedback since this is at least the publicly given rationale for the delay in shipping the EF.
Finally, and this one may not pan out, but it is my own hope that ADO.NET EF is being re-aligned somewhat with Linq to Sql. These two O/RMs are similar enough to share at least some code, and I believe that some of the MS guys have hinted at this too.
So I'm happy that at first there will be one O/RM -- Linq to Sql. The gurus may be disappointed, but the vast majority of MS devs will be new to O/RMs anyhow, and Linq to Sql will be good enough. Very much like my simple WilsonORMapper has been so widely used.