Moreover, my point was that the differences between releases are relatively easy to cope with -- yes, even for py3k. Sure, there are a hefty number of changes, but they mostly occur in a more "library" level (if you will) rather than at the base language, and much of it is just cleaning up inconsistencies. It's not as though the entire "16 years of Python" flies out the window with an overhaul like this. It'll be something of a pain in the ass to update code, but it won't make your collected knowledge obsolete, per se.

On a less contentious matter, Arc is also a tiny language. Its changes won't be all that drastic at this point. This is little consolation, as the aim is probably to build up a respectable library base. To that end, my overarching point is that you should learn Lisp in a more platonic sense (hardly a waste of time, break-prone versions aside). This can be done with pretty much any dialect, for the record. Libraries may become obsolete, but this sort of philosophical knowledge will not.

Whether this entails using Arc needn't be a grueling decision: it's a small, simple dialect (cf the responses in the aforementioned thread), and still a Lisp. The impression I get from the announcement is more of a heads-up "we reserve the right to..." thing. Besides which, changes to Arc entail, to a significant enough extent, code written in Arc itself. What better way to understand and play with the language than to read over the definition of 'trav (as a real example of a change between arc1 and arc2; http://arclanguage.org/item?id=2504)? Regardless, as Arc stands it's stable enough for learning, but this education has far more impact than being some sort of fleeting "woops, new version, guess I don't understand Lisp anymore." I focus on this idealized learning-experience viewpoint because that's the reason I tend to play with other languages: to expand my mind.

If, however, it comes down to consistency that you'd like for a Lisp in a production environment, well: