Just for comparison, Wii was announced (as Revolution) 18 months before it's release. The next system being announced at E3 would follow that perfectly for a Q4 2011 launch (and also follow Nintendo's usual 5 year console cycle).
Now, I'm not 100% they'll do something like this, but it's not exactly unprecedented or even unlikely. Nintendo tends to formulate their plans around Japan, and Japan is looking like it's very much in need of Wii refresh...










