Canon 85mm 1.8 for Macros
Interesting question. You certainly can do it, here's a gallery of samples :
However, even with a full set of extension tubes (65mm) you don't quite get 1:1 magnification (approx a magnification of 0.875 or 1:1.14). The EF 85mm f1.8 also shows severe CA on specular highlights - see the jewelry photos in the gallery and similar shots from a Tamron macro lens in a sub-gallery on the second page.
If you already have a set of extension tubes it's worth trying, otherwise I'd suggest saving up for a real macro lens.
Check out my galleries (lots of macros) at :