Howard,
>DDE comes to mind - have you used any others?
DDE is the only one *documented* to work asynch.
I've worked with
a simple semaphore table (good enough for most apps)
DDE synched and asynchronous
COM Exe's talking directly to each other

and prefer solution 1 if no timing problems exist (KISS as long as possible)
and 2 if extremely fast reaction is asked for - no external dependencies except DDE.

But Calvin Hsia has blogged this month twice on VFP multitasking,
so make SURE to look into that! I've only skimmed it as of now.
Also consider the poolmanager examples, Google should find you Remus'
MT-approach, and last but not least here are a few downloads here
on UT on callbacks and message forwarding -
haven't tried them, since I had already running approaches.

Since DDE is(?)/was a 16-Bit-technology, I'ld make *certain before*
implementing that this approach is still functional in 64-Bit windows,
since there at least *some* 16Bit code is rumored not to run anymore.

But ultimately your app's needs should govern your approach:
just abstract it enough not to paint yourself into a corner <g>.

my 0.02 EUR

thomas