Gmail's quota will continue to increase in the following years. Google updated the code responsible for the storage counter from Gmail's counter and revealed what to expect in the future:
2835 MB on April 1st 2007
2980 MB on April 1st 2008
3125 MB on April 1st 2009
3270 MB on April 1st 2010
3415 MB on April 1st 2011
3560 MB on April 1st 2012
As you can see, Gmail's storage will increase with 145 MB a year (that's almost 0.4 MB a day). Of course, if Google doesn't change its mind.
On April 1st 2005, when Google announced the "Infinity+1" plan, the storage increased from 1 GB to 2 GB and, from that point, the growth rate was 3.456 MB a day. But from October 2005, the growth rate dropped to 0.33 MB a day.