VISTA: will this improve Linux foothold?
Unlikely -- there are two current trends:
1) People holding on to XP as long as they can
2) Most of Apple's gain in userbase is coming from the Linux camp, rather than Windows.
So... Vista is slow on the uptake, people aren't giving up XP, and the most likely 'switchers' were already running a non-windows OS in the first place.
People may be hesitant to switch to Vista, they're *also* hesitant in giving windows up altogether... The huge market share of windows solutions combined with a ton of 'necessary' software they use that simply isn't available on Linux is still a deal breaker.
By comparison, the Mac is a less radical alternative. Still a minority OS, but at least major software suites like Photoshop and Microsoft Office are available for it, which makes for a much easier transition for a lot of people than it would be to switch to Linux.