I got Scribus 1.5 running in Ubuntu under Virtual Box in Windows 7. (For some reason I could only run the 32bit Ubuntu, although I have Windows 7 & Virtualbox in 64bits.)
The instructions provided by leaf didn't work at first (don't remember wich error I got, but something got wrong at the end).
I then installed the regular version (1.4.3) from the Ubuntu Software Center. That did go well.
Then I retried leaf's instructions and hurray; it worked!
But alas, for some reason there wasn't an option to save as pdf/x-1a, so it was of no use to me.
I didn't look any further for a solution because in the meantime the kind people at the printshop examined a pdf v1.3 output that I send them and that was perfect.