A recent report from prominent Indian newspaper indicates that Google has developed an offline version of Gmail that is likely to be made available within this year.
Not surprisingly this runs on Google Gears. Google has previously offered an offline version of Google Reader using the Gears browser plugin.
Although the story is unconfirmed by Google at this stage, it would be fair to presume that an offline version of Gmail happening is a given, the only question is when it will be available. Google continues to put together its jigsaw puzzle of office functionality as it builds a serious Microsoft Office competitor.