Yes, yes, this is exactly why I need to get myself a k5 when my budget allows! New possibilities!
Not just the low light + fast lenses, which you gave some great examples of above, but low light + slow lenses will benefit a lot from usable high iso settings.
I shoot birds, and as soon as my april salary comes in my husband has told me that I can buy a Sigma 150-500mm. I will be using it at 500mm most of the time, which means f 6.3, and my k7 hits my noise-tolerance-limit at iso 400. Which means that I need BRIGHT sunlight to capture any moving bird with my k7. And anyone who ever visited Denmark will know how rare bright sunlight is up here.
So the next item on my wish list is a k5. I just need the price to hit $1000, and my bank account to recover from the Sigma lens - those two events just might be well timed with each other some time around autumn!
Until then I just visit this forum to feed my envy. And you guys are doing a great job with that!