This sounds like a variant on 'I've got dibs' (or 'I've called dibs').


See e.g. the second positing for 'downs' on the Urban Slang website (but I don't think the explanation is quite right) and the Wikipedia article on 'dibs'


2. downs
3 up, 4 down


Downs: used to announce intentions of going next.
Origin: a term coined by the South Florida basketball scene but maybe applied to a wider range of usages
Usage: "ai nigga you know i got downs for next game, lemme get that gamecube controller son"

Tyrone called downs for the next street basketball game.
Tyree called downs on the library computer cuz he can't afford his own.

http://www.urbandictionary.com/define.php?term=downs
Dibs (also known as bags, bagsy, or bagsie) is a common convention used primarily in the U.S. and UK by friends or siblings to reserve or declare full or partial ownership of a community resource, such as a chair or communal food
http://en.wikipedia.org/wiki/Dibs

--------------------------------------------------
Note added at 2 hrs (2007-07-28 13:18:34 GMT)
--------------------------------------------------

Or perhaps more precisely in the given context, 'I've formally announced my claim to it'.

--------------------------------------------------
Note added at 4 hrs (2007-07-28 14:54:03 GMT)
--------------------------------------------------

Just musing: maybe this originated from American football, where the team with the ball keeps possession of the ball as long as it keeps 'making downs' (i.e. advancing by at least 10 yards in four plays or less)