Martin Dudok van Heel (the European reseller of Ettus Research products and a GNU Radio developer) brings some good news:

With the release of Quartus II 9.1 on 2 november 2009 Altera has finally
decided to make a free Linux Web edition available.

This means it is finally possible to synthesize the FPGA firmware for
the USRP on Linux without having to use a windows emulator.