DISTINCT ON () with UNION

How can you use a distinct on () including the whole union.

eg

select distinct on (valutaid) valutaid, short from valuta UNION select 
landid, land from land order by valutaid;

table: valuta
valutaid	valuta
1		USD
2		SEK

table: land
landid	land
1		Sweden
2		USA
3		Norway

The above would give
1	USD
1	Sweden
2	SEK
2	USA
3	Norway

What I would like to get is
1	USD
2	SEK
3	Norway

I did specify distinct on (valutaid) anyways =)

Any suggestions?

Best regards
Per-Olof Pettersson