GeoServer supports MrSID through the GDAL Image Format plugin but i have seen @iant comments here as:
I wrote a simple tutorial on how to build a pyramid data store in
it works with png imagery but providing you have a MrSid licence I
believe all the tools work the same way. Once you have the data in
GeoServer you can set up GeoWebCache in front of it (in version 2.1)
you don't even need to install anything extra. By default GeoWebCache
will cache in 4326 and 900913 so there is no need to reproject your
data if you don't want to.