[gcheong]

"there seem to be a lot of crap jobs out there, lots of people looking for experts in Java or .NET."

Choice of language has little to do with whether or not the job will be intellectually challenging or whether the company will provide opportunities for you to grow your skills. There are a lot crap jobs in Java and .NET because there a just a lot of jobs in those languages in general. So unless you are keen on using a specific language I would try to be as language/tool agnostic as possible. As to how you detect whether a job will foster your intellectual growth, try asking some questions along those lines in your interviews. For example:

"How far do you feel you have come in terms of your skills compared to when you first started working here?"

"What was the most challenging thing you have gotten to work on here?"

As far as trying to convince employers that you can pick up whatever they are using, my experience has been that employers asking for X years of experience in Y are not likely to be convinced and the likely-hood of changing their minds is probably proportional to the number of years they are asking for. So look for employers emphasizing fundamental skills over particular technologies because they already get it and it won't really be a question.