There's already a tip that describes how you can get the daily weather report in your Google Calendar -- http://www.tipstrs.com/tip/7785/Show-weather-forecasts-directly-in-your-Google-calendar.

For those using iCal, it's pretty easy to get the daily weather report in iCal too using the Weather Underground site. To do this go to their site (http://www.wunderground.com/) and search for the region where you want the daily weather report.

Once you get the search results, you'll see an iCal button near the top right of the page:

Weather Underground search results showing the iCal button

Right-click on the iCal button and copy the URL location.

Then with iCal open go to Calendar -> Subscribe... in the menu bar. For the Calendar URL, paste in the URL you just copied.

You can then choose a color for your new calendar event and well as the update frequency. Save your changes and the weather will then show up as an all-day event at the top of the days in your iCal calendar.