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.
For current conditions, you could try YQL, but I'm not sure if it's really current. (A good article: http://developer.yahoo.com/blogs/ydn/posts/2009/12/extending...)
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.