Transparency in PDF/X

If you design a print piece like it was a website in 1996, then just forget about outputting to PDF/X. Unless your printer is state-of-the-art, you just won't get a satisfactory print. Older workflows "flatten," or rasterize sections of your artwork where transparent elements overlap, resulting in obvious glitches and errors. When outputting transparency to PDF/X, you might expect some of these problems:

Ghosts or white boxes where the transparency effect should be.
Rasterized, or "jagged" text.
Artifacts such as hairlines or broken lines.

None of these problems might make themselves visible until you get the final print back, when all the money for the job has been paid. No one wants to be in that situation, especially with the expectation that PDF/X will take care of everything.

PDF/X is actually fairly complicated. As of this date, there are three versions:

PDF/X-1A—the most widely accepted format for outputting CMYK jobs. PDF/X-1A does not support transparency. Your file will require "flattening" before outputting.
PDF/X-3—for a color-managed workflow, where you might print outside the CMYK gamut. It doesn't support transparency.
PDF/X-4—This version suports live transparency. Only Adobe Creative Suite 3 and later output to PDF/X-4.

So, unless you (and your printer) are working with the most up-to-date versions of everything, then you are not going to successfully print transparency. Here are some suggestions, in descending order of preference:

Output your files in plain PDF 1.4 (Acrobat 5 and later.) This version of PDF supports live transparency.
Send your document as a ZIP package of application files and images.
Export each page as a TIFF file. Make sure this is of the correct resolution and color space.

Even with these options, ignoring the impact of transparency on your artwork is just lazy, sloppy design. Use the Transparency Flattener in Adobe InDesign to see what elements of your project will be affected by flattening.