Since we are using multi xap scenario from the first steps with Silverlight I would like to remark main features that it should support.
1. Multi xaps should share common dlls. Currently we are creating all xap via cached dll option and than while parsing xap manifest dowload assemblies zips by hand. Without this feature using of multi xap is useless. Example: You've got a Grid.dll (1Mb)
and you are using grid in 3 of 5 modules. So only first grid depended module should load that dll.
2. The second main problems is style sharing. I've already blogged about it (http://weblogs.asp.net/alexeyzakharov/archive/2009/10/14/issues-of-silverlight-style-system.aspx). Currently we solve it by our custom lazy style loader (if style related dll is
not loaded skip it and try next time). I hope that next Silverlight version will care about it itself.
PS: We have already created one project on our own framework which work in multi xap scenario. You can check it here: househotel74.ru.