Well as Rennie says, the main issue are the quality of the lenses (and I have a 16MP OM-D MD5 I'm trying righ now and it is tiny!), but Sony is working hard. They are naming their interchangeable lense full frame cameras, the A7 and the A7R and will announce on October 16th it is rumored. The main question will be the quality of the glass, but this could be some real competition for Nikon and Canon particularlyi as I'm using the EVF on the MD-5 and have to say EVFs are now pretty darn good!
An anonymous source (Thanks!) sent me the A7a nd A7 prices in Canadian Dollars:
1) The A7 with 24MP sensor will be $1700 Canadian MAP
2) The A7r with 36MP sensor will be $2300 Canadian MAP