I recently ran into a problem where after adding a new custom entity to a system I was unable to do a build in the application architect.  I was suddenly getting errors like:

C:\Documents and Settings\user\Application Data\Sage\Platform\Output\implementation\Account.cs(159,23):The type name 'Int32' does not exist in the type 'Sage.SalesLogix.Entities.System'

C:\Documents and Settings\user\Application Data\Sage\Platform\Output\implementation\Account.cs(159,23):The type name 'String' does not exist in the type 'Sage.SalesLogix.Entities.System'

It took me a second to figure out why I was suddenly unable to build.

Then it occurred to me.

I had created a new entity named "System".

Well in doing this, I inadvertently caused a conflict where they system was confused by the standard System class and my new Entity called System.

So a word to the wise.  Carefully consider the name of your entities.  Don't create them using the same name as Classes that will be used by the system (and by system I mean SalesLogix, not System :) ).