In SharePoint 2010 (SP2010) you can create a silverlight web part pretty easy by just creating a custom control, then deploy the XAP file to the clientbin folder. I should note that the pre-canned silverlight web part that I am using does not allow for DCOM bridges to be used, so to communicate between multiple silverlight web parts you would have to the Messaging API, which is quite easy, but that's for another day.
Getting Started, Open Visual Studios 2010, and create an empty SharePoint Project and a Silverlight Application. For the Silverlight Application project ensure that the "Host the Silverlight Application in the new Web Site" is unchecked.
Now we are cooking with fire, and we can create our Silverlight application. For this example, I will just add a map and one layer to the project.
Save and compile the project. If you get any errors, it's most likely you need to reference the proper ESRI silverlight libaries.
The next step is to configure the SharePoint Project. Add a new item to the SharePoint project and select Module. Double click on the Elements.xml file.
Configure the elements file to match the above image, you need to change the path of the Url to "_layouts/clientsbin/ XAPFILENAME.XAP" and change the Path from the textfile name to the XAP file name.
Click on the module and select the Project Output Refer -> Add a member -> Choose the Silverlight Application name -> in the Deployment Type Select ElementFile -> press OK
Select Build From main toolbar -> Configuration Manager -> Uncheck all the builds except for the SharePoint Project Name and make sure the SharePoint Project has deploy checked.
Save the Project and deploy to SharePoint.
Open your SharePoint site -> Edit Page -> Add Silverlight Web Part -> enter in the text box the following "_layouts/clientbin/XAPFILENAME.XAP". You should now see your web part.