Recomendations for a modbus library

I have just been presented with a little job.  I need to make a
converter between modbus protocol and a proprietry one.  They are
thinking of using the xnut [1] unit.  I would have to write the code to
make this convert the proprietry protocol to modbus over RS485.  Their
protocol is significantly different to modbus.

Xnut runs linux.  The xnut library does not support modbus (I think).

Are there any librarys that provide modbus support that I could use?  I
am fairly used to programming with linux, but not with embeded linux.
If anyone wanted to offer their services as a sub contractor I would be
very willing to listen.

Thnaks for any help.

[1] http://www.proconx.com/xnut/