09:59 Jul 25, 2011 

The absence of separators came about during the first era of data transfers. It was easier for computers to interpret empty spaces in financial DB files uploaded from non-English-speaking (not only Spanish-speaking) countries to update databases.

09:30 Jul 25, 2011 

That's right: it is permissible, but not obligatory, to omit the separator in numbers below 10,000. However, it is commonly done, and may be obligatory in certain publications, depending on their house style.

As for usage in Spanish, yes, the dot is now not approved by the Academy, though it remains almost universal practice, as I said. I have actually had it removed by Spanish copy editors. But current RAE rulings in the Diccionario panhispánico de dudas (from which I quoted) apply to Latin America too, because they have been agreed by the Asociación de Academias de la Lengua Española, comprising the Academies of all the Spanish-speaking American countries.

09:16 Jul 25, 2011 

It is not that numbers below 10,000 should not inlcude a comma - simply that it is permitted that they do not.... i.e. as Charles says 3,000 3000 or 3 000 (less typical is the last one).

09:14 Jul 25, 2011 

Asker asked what the rule is in Castellano - i.e. Spanish Spanish - S.American variations are not ruled by the RAE are they? They suffer a much bigger US influence..... ;@{

I just read in one of the refs below that the RAE actually recommends the use of a space rather than a "." in Castellano. The soup thickens.....

22:54 Jul 24, 2011 

You've also asked what the rule is in Spanish - surely, as a Spanish speaker, you know this?

22:13 Jul 24, 2011 

In English you need a comma (not a point),