Sadly, I think that it is a good observation that a lot of places don't respect anything you have done which doesn't look like it was done in an enterprise context. My view on this particular practice is somewhat mixed, though.
The first thing I think about is when I decided to quit teaching and go (back) into programmering. This was probably right after the bubble burst and the layoffs were going. My programmering skills were pretty rusty and I hit the same wall Stephan describes with every recruiter I talked to. It was quite frustrating. I sympathize with anyone in that position, to some degree.
Then a few things happened. First, I talked to engineers (sometimes with fancy titles) instead of recruiters at a couple places and landed a job at a small company which didn't use recruiters. Woot. Eventually I landed at another company which totally changed my view of software and economies, and I came around to seeing why open source really is a better way for a large chunk of software. That is a digression, however.
From there I have steadily moved in a different direction on my view of the "recruiter won't talk to me because my experience is [hobby|open source|user group|part time]". Really it was learning a little bit of economics (thank you Roy and Mike!) and changing my view of "work."
As far as I am concerned, every recruiter, HR person, hiring manager, or whatnot that feels this way is doing me a huge favor by weeding out a lot of motivated and smart people from their applicant pool. This has two direct effects, first it is an invisible damper on the company's capacity, second it enriches the candidate pool for companies I work for. Making your company easier to compete against is fantastic, as is making sure to not snatch up the most motivated candidates. That is my perspective from a business point of view.
From a purely personal point of view... well, I am colored by the fact that I have a reasonably good looking resume now. Most of the accomplishments that mean a lot, to me, on it relate to stuff I have done independently of my job description, however. Interestingly, my most recent work has grown mostly out of those things (hobby kind of stuff).
So, when confronted with this... places that eliminate you from consideration because you are self-taught are probably not good long-term prospects for you, though if you are not presently working you may not care about long term at the moment, so this is a bitter pill. More helpfully, maybe, look for places that don't recruit through technology check lists and resumes spammed from recruiters, it is more work on your part, but has a better success rate, I believe.
Oh yeah, on the "learned in user group" thing -- if you are active in the user group, tell people there you are looking for a new job. Smart companies recruit out of user groups, it is a self-selecting group of people who have proven at least a modicum of initiative and motivation.