Yes you can do this.  If you know how to build a function that does this for a form you can set the default value of a table to a function.

 so build a function
  public function getPK() as string
    'some function here to determine the user name
    'determine the last id in table
    'create a new key
  end function

in the table default value
  =getPK()

However if you are currently using an AutoID you can simply add another field for UserCreated in the table.  Set the default for that field to a function that returns the user.  There really is not a good reason to make the PK contain the user id.  Just let the PK be unique and have another field for user.  You can always concatenate this info for visibility.   

FYI.  I have not seen it yet but I have read that 2010 has table level events called data macros.  So lets say you wanted to track the last person working the record.  Currently you can only do that at the form level.  But data macross would be like you were asking were the event would occur whenever table is effected.  See here:
http://blogs.office.com/b/microsoft-access/archive/2009/08/13/access-2010-data-macros-similar-to-triggers.aspx

Anyone using these in 2010 have info/opinions?

MajP, thank you for your help. I will give this a try. Unfortunately we are still at Access 2003 but will definitely keep this in mind for the future. So much easier than changing forms all over the place.
Regards Alex

Alex,
  I think I gave you bad information.  I do not have Access on this computer to double check.  However, I know you can use a vba function as a default table value.  Example in a date field you can create a time stamp
 = now()

However, you might not be able to use a User Defined Function (UDF). You will then have to use vba functions for the default value. You may want to test
=Environ("username")
and see if the passes the user name.