OmniGraffle + LaTeX = Victory!
One of the very few shortcomings of OmniGroup’s peerless diagramming application, OmniGraffle is the lack of support for mathematical symbols and equations. You are essentially limited to the math symbols available in the character palette, or to importing equations generated by another application. I have created a simple Automator script that typesets LaTeX equations on the fly, using the LaTeX distribution installed on your system, and places the resulting graphic in OmniGraffle. To facilitate editing or re-generating the equation, the script also populates the notes field of the equation graphic with the original LeTeX code.
Be sure to put the file in the OmniGraffle scripts folder (select the Open Scripts Folder menu item from the Scripts menu, then select the OmniGraffle Scripts Folder submenu item). The script should certainly work if you have MacTex installed, and should work with other flavours of LaTeX, so long as pdflatex is in either /usr/texbin or /usr/local/bin. The only other dependency is the Python Automator Action, as the script is heavily reliant on Python.