It was a long time that I was wondering when the offline service will be available….
Gears is out for a while, so the technology was there….
Today I discovered gmail offline becomes available from the lab.
Well, it’s the same idea of the readers offline… It’s really good stuff. Give it a go. You need gears and lab installed 
How do I configure this? Do I have to install something, like a firefox extension or other?
Is it difficult to configure?
1. You need to install gears first,
2. You need to activate lab in your gmail account (Tabs lab)
3. Then you need to activate gmail offline in the lab features
4. Last but not least you need to start the synchronization. It takes a while at the beginning, then it will always synchronize automatically.
It works really well from my experience, you will able to access to your mail, even the attachments and to write and your mail and send them offline. (Of course they will sit on your OutBox folder untill you connect, and then will be sent straight away.
Thank you, I managed to get it working
If it were not for your post, I would have missed the others google/labs features for gmail, and many of them are very nice.
Glad it helped