I have already written my own version of an auto numbering plugin but today I saw that someone has created a codeplex project.
I had to rewrite mine because initially did using LINQ only to find you can’t use LINQ on CRM 2011 Online and then had to rewrite it using QueryExpression.
Looking at this plugin it seems almost identical (there is only so much functionality an auto numbering system needs) except my version has a pad facility because the users wanted to have the counter 6 digits long etc.
Go here and download the project if you want some auto numbering functionality in CRM 2011.
here is a brief description from codeplex
This is a solution for crm 2011 (hosted/on-premise) for autonumbering the configurated entity.
As an unmanaged solution, files included are:
1. A plugin for sequence generation : CRM.Plugin.SequenceGenerator
2. A custom entity for configuration: Counter
3. Three icons for the custom entity.