[FroshKiller]

This is also true in the US, although I would add that being down for something connotes a tiny bit more enthusiasm than being up for it, since "down" is also used in the context of making a commitment, e.g. "put me down for two boxes" or "I'll mark you down as present."

[evgen]

The latter case is related more to 'writing something down [on paper]' than to a standard up/down usage that implies directionality.

[drewzero1]

And certainly being marked down has different connotations from being written up!

[lupire]

Don't forget markdown and markup!