This was in the Adobe knowledge base:
http://www.adobe.com/support/techdocs/330714.html
9. Adjust the transparency flattening. (Acrobat Professional only)
If a document contains transparent objects, Acrobat flattens the document before printing it. Flattening removes transparency information and converts images to a format the printer can interpret. Adjust the transparency flattening to determine if transparency is causing the problem.
To adjust the transparency flattening:
1. Choose File > Print, and then click Advanced.
2. Choose Transparency Flattening from the menu.
3. Adjust the Raster/Vector Balance:
-- If you print to an inkjet printer, drag the Raster/Vector Balance slider to the lowest setting.
-- If you print to a PostScript printer, drag the Raster/Vector Balance slider to the highest setting.
4. Select Convert All Text To Outlines.
5. Deselect Clip Complex Regions, click OK, and then click Print.
Even though the note is for Acrobat Professional, it sounds like the behavior is something Adobe-related.
cynthia