MyMap.Extent = (dynamicServiceLayer.InitialExtent)
Yes you will need to use a GeometryService to project the envelope.
You could write the projection logic yourself on the client, which for Mercator is pretty simple. It's even simpler if your data is in WGS84 geographic coordinates (epsg:4326). In that case you can just use the ESRI.ArcGIS.Client.Bing.Transform classes.