Nope, ours went up - even had a spike upto £90 for one day!
Didn't get anywhere near that for the rest of the month though, still had the best month so far though. I've found the income pretty steady really. But I'd imagine it depends on the adverts shown on your site, more competition = more money for adsense.
Perhaps - I don't know, I just know that I wish I'd put adsense on a while ago. One very, very interesting thing I did last week after I found out you could do it - add a custom variable into your Google Analytics code. I've put a bit of code in that finds out if the visitor is a Guest, a Member or a Supporter (i.e. paid member) and stores this in the Analytics feed.
Now in Analytics I can get stats for each of the 3 user groups - including Adsense revenue figures!
Very eye opening and indentifies where you get most of your revenue from. In my case, Guests and shows that I almost certainly could drop the Adsense for supporters and lose only a few cents in revenue. I'm going to leave it going for a few weeks to get some decent data before I do it though.