I'm accessing a SharePoint 2010 implementation that has been setup by someone else and I don't have admin access to explore further. I also don't have a multi-farm VM setup available to test on at the moment.
There are 3 farms: ASIAPC, EMEA, and AMERICAS. Users can create a My Profile only. This will be be located on the farm that is closest to them. If they are on the ASIAPAC farm and they try to tag something on the EMEA farm it doesn't appear on their profile. If they tag something on the ASIAPAC farm it appears on their profile page.
Is this the expected behavior? When you tag something and return to add more tags the existing ones are listed, but the links below "Recent Activities" on the team site add tags popup point back to the My Site URL on the EMEA farm. Following these takes you back to your canonical My Site URL on ASIAPAC. I assume the redirection is caused by the Trusted My Site Host Locations setting.