Mikey85, I am not pro but i can surely tell you change in the title tag has a huge effect on your SERPs as well as Adsense earnings.
Atleast it made quite a difference for me (on a better side) when i changed the title tag of some of my sites.

Rajiv

hmm.. I changed it from:

Productname

to:

Productname - For sale in Categoryname

I've seen a 20% decline in CTR, and another 33% decline in traffic. Earnings have, for this reason, definitely dropped!

Have 1000 pages of uniquely written content and things were just going great. Too bad, nothing I could do to reassure Google that we aren't those bad guys!

Took a positive approach now and have begun adding 4+ articles everyday!

What changes did u make to the title tag?

I would kind of doubt that you would get that much of a drop in revenue (or see the ads drastically change) because of a change in title tag.

Mikey, I shorten the title little bit. I mean what I did was earlier title was like "blue widgets from blah blah blah in city abc country" Now we changed it to just "blue widgets" on most of the places.
In my experiment I felt more precise Title, the better. I mean if your page is about "ProductName" better to have title as "ProductName" than to have "ProductName -- Category -- City" or whatever.
I have seen many times this gives a good effect in adsense.
Caution: Results can vary in every industry where there may be very less advertisers for "ProductName" so irrelevant ads may appear.

Rajiv

your pages quantities are huge. Wish you good luck. Do you have a good income like before.

@rajiv...

I definitely think title and such things matter.

It all depends on relative merit.

"ProductName"

"ProductName -- Category -- City"

Now in the first category, competition will be more, so unless your ranking is higher you would not get much traffic.

However in the second category, you will have usually lesser competition since it is more specific. So you have chances of getting better ranking from it.

Now it is about trends. Go to google trends and find out which way more people search... the first way or second way.

Now it is total people searching a particular way plus your site's ranking on that particular search that is going to affect the way people reach and click.

If your site is very well ranking on "Product Name" then you dont have to worry about bigger titles, else usually lesser the term bigger is the competition and bigger the term, lesser is the competition.

Did I make it confusing and complicated :)

@doubtmaster:

Did I make it confusing and complicated :)

Not at all.
I think what you have written makes perfect sense. I tried various combinations and checked the effect for few weeks before coming to conclusion for MY site.
Sitewide things will be different ofcourse.

My observation was that if you go like "ProductName -- Category -- City" then many times google dont have the ads for productname or category it is showing irrelevant ads about the City and anything added to it.
For eg. City Jobs, City Car hire etc. So in my case it worked with shorter title. In many sites it may work with longer title. So no set rules for it thought. Its always about trial and error method we use.
Also the OP was not talking about Ranking but it was about Adsense Earnings going down after making changes in the page including title.
Ranking will have somewhat different effect because of title. But then you have to find a delicate balance for your ranking and for adsense.
Ofcourse one thing I think we both agree is that title plays a very important role in which ads will be shown on your page. Right?

Rajiv