After much Googling, I discovered that some Unix commands create their output by writing to standard error (STDERR) instead of standard output (the Terminal window, typically). As a result, operations that capture standard output won't work for these types of messages, even though the message itself is displayed in the Terminal window. I found the solution in the Wikipedia entry for stderr: append 2>&1 to the end of the command (note that this is for sh-style shells, including bash, the OS X default shell). This redirects standard error to the same destination as standard output, meaning the data can then be captured as you wish. To put the output of httpd -t onto the clipboard, for instance, you could use this:
$ httpd -t 2>&1 | pbcopy
$ pbpaste
Syntax OK
Or if you wished to assign the output to a variable for use in a script:
CONFIGTEST=`httpd -t 2>&1`
I'm sure there are a number of other commands that output via standard error -- one for certain is ssh -V, which returns the version information for ssh.

