I've attempted to get some worthwhile information by looking at the question and answer trends over the past two years. I am looking at how things such as average answers, accept rate and answered questions are trending over time.
The data is slightly skewed because answers provided this month can apply to questions asked in a previous month. This means that over time the numbers for a particular month will change and things like avg. answers per question in a given month will always increase slightly over time. However I think the overall trend still speaks for itself and is still a wortwhile indicator. I would argue that an asker is more likely to accept an answer in the first month (or two) and you would suspect a question would receive most of its answers in the first month (or two) of its existence.
The query used to obtain this data is available here. I have simply exported the data into Excel to create the charts.
Looking at the average answers per question over MM/YY we have gradually trended down from 4.3 in January 09 to 2.01 to January 11. Note that with new Area51 sites we are setting 2.5 per question as Good and 1 per question as Worrying.
If we look at the number of questions and answers over MM/YY we can see an increasing trend in both questions and answers. I have included in the chart below the line for (questions x 2.5) which shows where we should be with answers if we were seeking an average of 2.5 answers per question.
The last piece of data I wanted to show is the percentage of questions with an accepted answer (on the chart as AcceptedRate) and with at least one answer (on the chart as AnsweredRate). SE uses a the following definition for "Answered": An answer has to be either upvoted or accepted in order for the question to count as "Answered". According to Area51 90% is a healthy Beta and 80% is worrying. According to the SE definition SO is currently sitting at 82%.
This is a more strict definition than a question having a single answer for which we are trending at around 86/87%. For accepted we are sitting around 53%.
I don't want to make any sweeping statements about the data. But I think says something about where we are heading.