Quite some time ago, Accord.NET had been upgraded to .NET 4.0. This upgrade was made to advantage of some of the unique platform features. However, this was a drawback for those users who were still bound to .NET 3.5 and would like to use (or continue using) the framework.
So, some weeks ago I did finish creating a compatibility layer providing .NET3.5 support for the newest version of the framework. While some features had to be left aside, most of the framework is currently functional. A few features, however, had to be removed. Those include the Accord.Audio modules, support for CancellationTokens on processor-intensive learning algorithms, some parallel processing capabilities and some of the Expression Tree manipulation in some numeric optimization algorithms and in Decision Trees.
With time, some of those could be re-implemented to work correctly under 3.5 as needed. You can leave a request, if you would like. Stay tuned!