Have your host install mod_geoip on your server and then put this as your .htaccess file.
Code:
GeoIPEnable On
GeoIPDBFile /path/to/GeoIP.dat
# Redirect one country
RewriteEngine on
RewriteCond %{ENV:GEOIP_COUNTRY_CODE} ^CA$
RewriteRule ^(.*)$ http://www.canada.com$1 [L]
# Redirect multiple countries to a single page
RewriteEngine on
RewriteCond %{ENV:GEOIP_COUNTRY_CODE} ^(CA|US|MX)$
RewriteRule ^(.*)$ http://www.northamerica.com$1 [L]
http://www.maxmind.com/app/mod_geoip
