Recently I've had to look at the recent chat activity of some users (moderation/etc). The chat profile links to the parent site profile, but none of the SE sites (including SE.com) seem to link back to the chat profile. Which means there is effectively no way of finding the chat user (knowing the SE.com user) unless you open all chat users with the same username and verify the parent profile userids.
Just now I had to move a discussion to chat. This involved creating a chat account for a 1-rep user, and then giving him write access. But: his username was extremely common, and there was no way for me to add him without knowing his chat id. Which meant I had to go through pages and pages of users with similar names to find him. Not. Good.