I was thinking about this earlier. a few more questions to add to the mix...
how, and out of what would you make them out of?
How do youdo it? start at the bottom and go up? how do you figure what size cutter you need to get them all on the cake so that you dont half a shape at the top, or bottom, or does that even matter?
Im not trying to hijack your thread, I promise, I jsut thaught you might have these questions to 
Huggs
Val
I've always wondered how people got "smooth" buttercream in "areas" or "shapes". I would think you could certainly do it w/ buttercream or fondant, but I would think fondant would be easier - just cut & lay on cake. Of course, you could also use royal icing to make a bunch of diamond-shapes. Not sure how big you are making the shapes or exactly what the pattern you want looks like though. Just some ideas - and some more questions!
I have made templates just out of a heavy paper. The good thing about making templates is that you can make one after you have your cake iced and you can make the sizes in relation to the size of your cake. No matter you do there will always be a half diamond at the top or bottom--unless someone knows something I don't. I have neve been able to do it without having a half. If you look in my photos, there is my purple wedding cake that has fondant diamond cut-outs on a BC iced cake, then one of the baby shower cakes has a diamond impression in BC and then my daughter's princess cake had the diamond impression in the purple fondant. HTH!
Oh, and I forgot, if you have square cutters (cookie or otherwise) and don't care if you don't have an elongated diamond, you can use those as well. This is a cake I did using the large Wilton square fondant cutter and just piped over with dots.
http://pg.photos.yahoo.com/ph/missyekelley/detail?.dir=ce37&.dnm=4dfc.jpg&.src=ph
Hello sugar friends this is how I do mine. At Walmart they sell car floor mats yes car floormats that are the clear plastic ones they have a diamond pattern I cut out the square with only the diamond pattern on it and aslo the bumpy part that holds the mat to the carpet can be used as a template for basketweave. anyways once the icing dries just put up against cake and lightly smooth with hands. Its plastic so I assume it is safe to use. Just wash real good before, it is only on the cake for a quick min or so. Hope this helps.
So I can just cut a diamond shape out on someething, and after the iceing has crusted I just press it on the side and work from there? Im trying to picture this in my head and im haveing trouble. I want to use this on a cake Im makeing this weekend. Since I dont really know what im doing yet I am trying to come up with simple techniques to make it look nice. Is this something you would suggest for a beginner?
]huggs
Val
Sorry! Sometimes I'm clear as a bell in my head, but then it all comes out fuzzy! 
When using templates made from paper, it really is more for cutting fondant pieces to put on the cake--they don't do well for impressions, unless you did it out of good stiff cardboard? BUT, you can always make a template, then place a piece of wax paper over it and make an impression like tool out of royal icing. So, draw a diamond, pipe a nice daimond shape in royal, let it dry and then use that piece to make the impression in the icing (though I'm not sure how well that would work with fondant). These were all little things I did before I bought my diamond cutters.
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