A simple solution would be to use commandline and delimit the string with single quotes.
<exec program="ruby" commandline='parse_text.rb "some text file.txt"'/>
However, this solution feels like a hack and isn't immediately obvious.
Another (even less obvious) solution is to use ".
<exec program="ruby" commandline="parse_text.rb "some text file.txt""/>
This doesn't read very well, but it feels less like a hack because you can use a standard double quote delimiter.
Either option you choose isn't very pleasing. Which leads me to reason #1 why I prefer Rake to NAnt.