Gav,
You have to stretch astrophoto's enormously so vignetting becomes far more obvious. Luckly, there are some good techniques (eg flat fielding) to deal with this, although its preferable to have even illumination to start with. I saw some comments on the 135 f2 vignetting from Christian Buil's site (
www.astrosurf.com/buil ) and photozone. The vignetting is gone by f2.8 anyway.
Gav, I seem to remember you had the 300 2.8 IS. Are you happy with that lens?
Terry