Can you create an example, with some dummy text, and post both .sla and resulting .pdf files showing the problem? I haven't noticed any problems with Myriad font, so I'd really like to see what the issue is (and the answer might actually be quite simple: an ancient version of Scribus; try upgrading to 1.4.0.rc6 and see if that worked in the first place).
In the attachment are two examples (created with version 1.4.0.rc5 on Windows), both using Myriad Pro OpenType font, subset and embedded, and exported with "Output indended for: Printer" in "Color" tab. I don't see any problems with display in Adobe Reader X. Please check.