Silverlight supports only a subset of .NET and is binary incompatible, which is quite a PITA. Microsoft made some seemingly random (I'm sure there are reason, but often I can't find them) decisions about what is supported in Silverlight. On top of that some stuff actually changed, e.g. lots of classes are suddenly sealed, non-generic collections are made internal etc.
As a quick check I converted the log4net project file to Silverlight and tried to build it, which resulted in quite a few errors. At least there are the following issues:
- Serialization needs to use DataContract, not Serializable.
- All collections need to become generic.
- XML parsing needs to be done useing Linq2XML as XmlDocument/XmlElement etc are not available.
- Some appenders depend on stuff unavailable in Silverlight (they could be disabled though).
- Lots of appenders will only work when elevated trust is available.
I also guess some of the required changes might break conflict with compatibility with older .NET versions.
I'd really like to have Log4Net available on Silverlight, but getting there won't be a trivial task.