I find that I can also successfully copy transformed text from the free (open source) Inkscape into OmniGraffle.

Note that to get a vector (rather than bitmap) copy into OG, you need to Save As PDF (using the 'convert texts to paths' and 'export area is drawing' options), and then use OG's File > Place image to bring the PDF file in.

Inkscape installed without problem (takes a few minutes on first launch) on my OS X 10.6 system. There is a different build for OS X 10.7, which I haven't tested.

http://en.dev.inkscape.org/download/

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