Just discovered this while preparing a conference poster.

Mac OS X.2.6. Imported a CMYK tiff with an alpha channel into FH MX to
place on a coloured ground. Everything looks fine on screen. Print to an
HP Colour Laser 4550‹where the Alpha channel mask is there is a subtle
colour shift on the coloured ground! So, to see if this was a printer
problem I printed to postscript using the distiller PPD and created a
PDF using Acrobat 5 in Classic. Sure enough the PDF shows the same
colour shift under the alpha mask!

I had to recreate the poster using InDesign were everything worked
perfectly, but cost me about 4 hours!

I have informed Macromedia of this (to me) major bug and have warned
them of my intention to take a serious look at the next incarnation of
Illustrator!

Les