This is a very nice feature, but I think I still prefer http://www.wunderground.com/auto/wxmap/ This page also has the ability to see where percipitation is currently. I find that this helps with my bike commute in Portland frequently.
It's interesting to see they're pre-rendering the composition of the weather layer on top of their regular map. Where I work we have our own styled map tiles and probably would do a layer of transparent cloud tiles over our (unmodified) regular tiles. It just takes too long to re-render deeper scales.
That said, if I was serving up as many tiles as google does, I'd go for it this way too. You already need a buttload of resources just to handle the request volume, might as well exploit that to save the user on download time and yourself on bandwidth.
What I'd really like is a weather API - I just spent 2 hours looking into a decent one to use. (The National Weather Service doesn't seem to report the current temperature. :/)
I still haven't been able to find an API to get yesterday's weather. I've been trying to make a site that doesn't show straight up temperature but a percentage based on known value (yesterday's temp).
The NOAA service (http://www.weather.gov/forecasts/xml/rest.php) that I've used before just does time series, so you might be able to get the most recent time, but you're right, I haven't been able to find a NOAA API for current data.
My educated guess is years from now if ever. There's no worldwide severe weather dataset, so any foray into that market would look half-baked or worse, biased towards first-world countries (ie nations with good metereological data).
The thing about weather sites is that they usually overwhelm users with information. 80%+ of the time, a user wants to know, will i need a coat or umbrella today? Or the general temperature.
