It's very very hard to differentiate people who have a passion for doing good work and people who just like to play with the latest trendy technology. Especially since the latter is often just an immature stage of the former.
This is actually a big reason why non-trendy technologies persist - indeed, why they often thrive in the larger marketplace precisely at the moment that they become least sexy and fall off the geek radar. Non-trendiness is a market signal of its own. There's a lot less noise in the backwaters, and those products which survive to become boring have demonstrated staying power. People obviously aren't using them for fun, or because they're easy to promote on popular blogs, or because VCs can impress each other by dropping their names. They must have survived for another reason. Perhaps they are... pragmatic, useful, or cost-effective?
This strategy must be used in moderation, of course. The final stage of nontrendiness is obsolescence and death; You don't want to hire those who are expert only in obsolete things. (Oh, my IE6-CSS-hacking skills will soon have no market value. Thank goodness!)
I'm always impressed by developers who systematically explore new technologies as part of their "non-commissioned" time. I'm a huge proponent of this.