Google Adsense just introduced a new ad unit that might give you an increase in your adsense earnings due to its overall huge attractive dimensions.
This new ad unit comes with the same width like your old 300 x 250 unit but now its length is increased to 600px making more wide impressions and more gain in visitor clicks.
I just opted for this ad unit and you might be able to see it on the right hand side in the sidebar, well I just came to know about it from one of fellow blogs and I thought that he is using some CSS hack to join two units to create one big unit.
But then I just thought to login to my account and see if there is any updated unit that I can use on my personal blog and I got this unit and it simply looks like a unit from a premium account but now its available for free to everyone, and you can use it on your any website.
So I will recommend that anyone who is using 300 x 250 ad unit from adsense should migrate to this 300 x 600 ad unit, so that he can gain some more clicks. Now I have even posted about some good placements for adsense ad units for optimizing your overall income you might wanna check that too.