In addition to the declining growth rate, the latest State of the Twittersphere shows that the average Twitter user is more engaged.
Consider the following changes:
Today the average Twitter account has 300 followers; in July, it had 70
The average account now follows 173 accounts; in July it was only following 47
The average account today has posted 420 updates; in July that number was 119
Why Twitter Applications Don't Affect This Data
The State of the Twittersphere measures
Twitter user growth
traffic on Twitter.com
The growth rates are calculated using the account creation data from over 5 million Twitter accounts. This data is platform agnostic -- it is the same regardless of the application the user uses. In contrast, measure of traffic on Twitter.com does not count the considerable use of Twitter via third-party applications.