In answer to Eric's post about the Argentinian cover, here's a bit about those security peforations, for, yes, that is what they are. They were introduced in the early 1990s as an extra security measure against stamp forgery.

The type seen most is the elliptical perforation. The shape of these differ somewhat (depends on who has produced them) and they have received fancy names such as "sausage" and "rugby ball".

The Argentinian stamp on Eric's cover has a rather intricate variety of the ellipse; it is more like an accolade. I haven't seen these on any other country's stamps.

A slightly more subtle (and therefore more effective?) security perforation is the larger corner perforation. You hardly notice the difference on single stamps, but the perforation is very clear to see in blocks. I have only seen this type of perforation on the Dutch Beatrix stamps and the Hong Kong Machins. Both were printed by Enschedé in Holland and may well have been discontinued by now.