maybe this can be set as an option somewhere?
Under the default language in perferences, i would have thought this would indicate the seperateor for the currencies?
if not, maybe it could be added there. I think its fairly standard isnt it? USA use comma's, Aus we dont use them, (although, looking at netbank now, they are.. hmm).
I like your solution - letting the user define it, or setting it upon user's nation.
Another option, which is not as good as yours (that I favour more), is to write the number also in letters...
"25250" - "Twenty-five thousands and two hundred and fifty".
[I'm adding this only because maybe setitng a new preference value would not be that easy or wanted to be implemented..]