I'm pretty sure I've seen this before and I just can't think of how to do it.
Say I wanted to check out which cities visit Google.com the most, how would I do that? Been scouring AdPlanner, Quantcast, Alexa and Compete but I think I'm having a blonde moment or something.
Don't tell me it was all just a dream 
Thanks in advance.
You were not dreaming mate (although it would be a very Perth-like thing to do
)
http://www.google.com.au/trends?q=google.com
also check out clues.yahoo.com
you cant do domain searches, but if you use keywords driving most volume, you can drill down to city level.
http://clues.yahoo.com/#q1=farmville&q2=cityville&ts=2
looks like yahoo have flicked back to the old version of clues cause i cant get global and different country searches anymore.