What do I need to know/do to write a database w/ FM6 on OS X and deploy it on a PC with FM6?

Font widths - Make your text boxes larger (wider) than necessary, or parts will get cut off.

Cool -- but what I want to know is how once the db is finished in OS X, how can I deploy it on a PC so that it will open with FM6 for PC?

You don't need to do anything special. FileMaker files are a cross-platform format. There are some minor layout differences, such as fonts, so use a cross-platform font; for screens I use mostly Verdana 10 or 11, with Arial thrown in (for any really small text).

For Printing I tend to go with Times New Roman. Printing Setup can be a little bit of problem, in that any special setup, like Landscape mode, you do on the Mac will not transfer to the PC. You have to open the step on a PC and reset it. This is much easier with version 7, because it explicitly shows you what it is.

PC's also handle windows differently. On a Mac, a window does pretty much what you tell it to. On a PC, the operating system has different modes for windows (all windows of a application), with the usual being to control the whole screen area (Maximize). If you Toggle Window [Zoom] in a script, the mode changes, which would be fine, except it creates an extra menu bar, which eats up quite a few pixels. So you want to either avoid Zooms, or keep those screens a little "short," then Toggle Window [ Maximize ] to return to full screen mode. Yeah, it's annoying, but can be dealt with in some kind of acceptable way.

You should definitely test your solution on PC, just to see how the above layout stuff plays out. Usually a few tweaks will take care of (or minimize) any glitches.

(P.S. If at all possible, you should develop in 7. It's so much better. But if they insist on 6, what can you do?)