So a while back I whipped up a thing called "springboard" for Google Apps/AppEngine (having got bored with maintaining text files for use with Apache's mod_rewrite) which allowed users to create redirect URLs like http://go.example.com/promo (and which was apparently a good idea because now Google have their own version called short links). This is the way forward - you can tell at a glance who's behind the link from the domain and you even get an idea of what you're clicking through to from the path (provided you're not being told fibs). When you click on this link you get flicked over to the real (long) URL with a HTTP redirect, probably a 301 which means "Moved Permanently", so the browsers know what's going on too. If your domain goes down then chances are the target will be out of action too (much the same story as with third-party DNS) so there's a lot less risk. It's all good news and if you're using a CMS like Drupal then it could be completely automated and transparent - you won't even know it's there and clients looking for a short URL won't have to go ask a third party for one.
So the problem is that nowdays you've got every man and his dog wanting to feed your nice clean (but long) URLs through the mincer in order to post them on Twitter. Aside from being a security nightmare (the resulting URLs are completely opaque, though now clients like Nambu are taking to resolving them back again!?!), it breaks all sort of things from analytics to news sites like Digg. Furthermore there are much better ways to achieve this. If you have to do a round trip to shorten the URL anyway, why not ask the site for a shorter version of its canonical URL (that being the primary or 'ideal' URL for the content - usually quite long and optimised for SEO)? In the case of Drupal at least every node has an ID so you can immediately boil URLs down to http://example.com/node/123, http://example.com/123 or even use something like base32 to get even shorter URLs like http://example.com/3R.
So how do we express this for the clients? The simplest way is to embed LINK tags into the HEAD section of the HTML and specify a sensible relation ("rel"). Normally these are used to specify alternative versions of the content, icons, etc. but there's nothing to say that for any given URL(s) the "short" url is e.g. http://example.com/3R. That's right, rel="short", not rel="alternate shorter" or other such rubbish ("alternate" refers to alternate *content*, usually in a different mime-type, not just an alternate URL - here the content is likely to be exactly the same). It can be performance optimised somewhat too by setting an e.g. X-Rel-Short header so that users (e.g. Twitter clients) can resolve a long URL to the preferred short URL via a HTTP HEAD request, without having to retrieve and parse the HTML.
Another even less sensible alternative being peddled by various individuals (and being discussed here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here and of course here) is [ab]using the rightly deprecated and confusing rev attribute ala rev="canonical". Basically this is saying "I am the authorative/canonical URL and this other URL happens to point here too", without saying anything whatsoever about the URL itself actually being short. There could be an infinite number of such inbound URLs and this only ever works for the one canonical URL itself. Essentially this idea is stillborn and I sincerely hope that when people come back to work next week it will be promptly put out of its misery.
So in summary someone's got carried away and started writing code (RevCanonical) without first considering all the implications. Hopefully they will soon realise this isn't such a great idea after all and instead get behind the proposal for rel="short" at the WHATWG. Then we can all just add links like this to our pages:
<link href="http://example.com/promo" rel="short">
Incidentally I say "short" and not "shorter" because the short URL may not in fact be the shortest URL for a given resource - "http://example.com/3R" could well also map back to the same page but the URL is meaningless. And I leave out "alternate" because it's not alternate content, rather just an alternate URL - a subtle but significant difference.
Let's hope sanity prevails...
Update: The HTTP Link: header is a much more sensible solution to the HTTP header optimisation:
Link: <http://example.com/promo>; rel="short"