Bruce,
It is, as a practical matter, a de-facto standard, and the specs for the various RTF versions are available from Microsoft (not Micro$oft in this case, 'cause the specs are free). It is listed as a standard on the UK JISC site, for example, and it has its own MIME type (text/rtf).
That said, the various implementations of RTF in the various products that use it are inconsistent, and there isn't (as far as I know) a minimum set of features that must be implemented to conform to any "standard implementation" of RTF. As a result, a given RTF file may (and often will) look different in Word vs. WordPad vs. OpenOffice vs. AbiWord vs. the Microsoft RichTextEdit control, etc., and transferring between the different programs/components may result in document formatting changes.
It is often, however, "close enough for government work" (as they say), and programs that read and write RTF abound. And haven't you ever had problems with web pages that work fine in IE but not in Firefox? So even "standard" (as HTML 4.01 is) doesn't guarantee complete compatibility.
Fred
