With new CRM 2011 Early bound classes (OrganizationServiceContext) coming into picture, it is now becoming a trend to keep your CRM API communication in a different library than your Plugin library.
Because most commonly the plugins are hosted inside the database, it becomes necessary to merge the communication library with plugin library. In order to setup your debugging information properly into the merged pdbs make sure you use the /ndebug attribute. The command below works fine for me and I was able to get my breakpoint into plugin and communication libraries
ILMerge Syntax
ILMerge /ndebug /keyfile:crmsn.snk /target:library /targetplatform:v4,"C:\Windows\Microsoft.NET\Framework\v4.0.30319" /out:Merged.dll /attr:CRM.Extensions.Plugins.dll CRM2011.Communication.dll