I tried ordering a channel entries tag pair by a playa field (that is single entry relationship), like so:
{exp:channel:entries channel="channel_name" orderby="playa_field_name" sort="asc"} looped stuff goes here {/exp:channel:entries}
What I got was in some sort of order, but it wasn't alphabetical according to the titles of the related entry. I think it's sorting according to the information hardcoded into the database for that playa field, which would sort it according to the entry_id of the related field because the text in the database starts with the entry id in brackets. This is my theory anyway.
Surely I can't be the first person to run across this problem. Any thoughts?