According to some newspaper resources, Google has already developed an offline version of Gmail that is likely to be made available within this year. 
Like Google Reader which lets you work offline using Google Gears, GMail users will be able to interact with their GMail account even when they don’t have an active Internet connection. For instance, the report says that you would have to download a GMail software client for that would automatically synchronize with the Gmail server. Read more here
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