I would say that "Scene" clothing is bordering on mainstream now days, with huge numbers of teenagers dressing that way. For certain, urban clothing has changed recently to a more polished style. Even preppy clothing has evolved with the popularity of AE and holister and younger teens embracing it, it's fresher now and less perfected. It looks like the influence of "surfer"/"skater" clothing has been minimal (perhaps because of the increase in scene dress). I like the trend of the last few years, more solid colored clothing - less screened tees and bottoms (less writing on one's bottom!).
I don't think fashion is as big a deal now as it was in the 70s and 80s. It doesn't seem to be as defining. Maybe it's because teens/young adults now have so many other gadgets to spend money, and so many other ways to express their personality and style. Most kids probably spend more time perfecting their myspace than their outfit, because they think it represents them better. I don't think that's necessarily bad, just different.
I also think fashion is always going to be more volatile with the younger crowd. So if you were a teen in the 70s you probably noticed huge changes in the coming years, but maybe not so much 20 years later.