I've been doing this for quite a while now.  Version before 2005 did not support extended properties for tables well.  I wanted a unified system.  So:

CREATE TABLE [doc_Fields] (

  [TableName] [varchar] (50),

  [FieldName] [varchar] (50),

  [Comment] [varchar] (4000)

) ON [PRIMARY]

I then put my field and table descriptions here.  If the field name is blank, or NULL, then Comment applies to the table.  A little grid based program to make the entry easy and one to generate an RTF file.

Now ported to .Net and using the extended properties it serves us well.  The advantage of the extended properties is that when I generate create scripts the comments come along.  This is also a help to the UI teams.  They can use this to generate tool tips.