Since Yahoo! Weather Module for Google Homepage had released in this morning, there are a few responses to me at Google Modules. One bug was identified by Rhyspace regarding temperature unit. The symptom is unit was always Fahrenheit even though the RSS URL was set to Celsius.
I have finally found the cause of this problem. It is the limitation of Google Homepage API that user preference must not contain value with "&" charactor. Otherwise, all characters after "&" will be discarded.
To fix this problem, I added a new user preference to explicitly specify temperature unit so that the missing unit could be ignored. Please wait for a while until the updated module will be published in Google Base.