I think an Ubuntu or Ubuntu's MID would be excelent although with out the name recognition(yes we might know what it is but general public has no clue). I think Android as an OS would help them out 200 fold.
if they replace symbian with anything it will be their Maemo OS.
That will not happen, at least not the way you said it. Nokia already has Maemo, an OS based on Debian and Ubuntu is a fork from Debian. Maemo is optimized for ARM (mobile) processors and Nokia has put money and develpment on it and I don't think they will be ditching that for Ubuntu, makes no sense.
Look for Nokia N800 and N810. People say it will be on Nokia N900, a new model which is some sort of hybrid between a tablet and a smartphone halfaway between N97 and N810.