>>20
In Scheme the standard is called the ``report'', you can find its different revisions here: http://www.scheme-reports.org/
A program that uses only what is in a given revision of the report will be portable among implementations that conform to that report. However, many implementations also provide additional features, that are unlikely to be portable. I don't know about portability among different revisions.
If you are a programmer by trade, you will end up learning many languages during your career and shouldn't worry about learning more than one Lisp. Generally, learning languages is good practice and will make you a better programmer. (See, for example, here: http://lambda-the-ultimate.org/node/3464 ) Scheme is a small language, you don't have to sacrifice decades of your life to learn it, if you are interested in it, give it a go.