Quoting Maxim Shemanarev <mcseem@...1076...>:

I'm afraid nobody has ever thought of it. Are there any recommendations from W3C?

According to SVG 1.1, circular references are an error and should be treated as such -- see the appendix on implementation requirements (F.2 Error processing). It's also referenced in the sections on <use> (5.6, just after the examples) and in the overview section of "References and the 'defs' element" (5.3.1).

Any problems with SVG implementations due to circular references is an issue of non-conformance to that normative appendix.

Inkscape gets that part of the standard partly right: it at least refuses to render circular references; in general they do not cause harm (if you can find some cases where it does, please let us know).

-mental