If you sign out of Gmail, you'll notice that the storage doesn't increase. It used to increase with about 0.33 MB a day, but the Gmail team forgot to add the code that manages the quota increase after 31st December 2006.
On April 1st 2005, Gmail space increased from 1 GB to 2 GB and started to grow every day. They called this: "Infinity+1 storage plan".
"Storage is an important part of email, but that doesn't mean you should have to worry about it. To celebrate our one-year birthday, we're giving everyone one more gigabyte. But why stop the party there? Our plan is to continue growing your storage beyond 2GBs by giving you more space as we are able."
Update (January 3rd): Everything is back to normal. On April 1st 2007 we'll have 2835 MB.