What countries are already represented in the Bitcoin network?
If you are interested in Bitcoin nodes, This website seems to keep a count of how many of them are connected by country:

http://bitcoinstatus.rowit.co.uk

Or, if you want more graphical (and cooler looking) data, just look at this globe:

http://www.weusecoins.com/globe-bitcoin/

It shows you what amount of Bitcoin nodes are connected in each geographical area not only at a country level, but going down to what I believe is a city level.