Nice question. Obviously several people might learn something since "System base table" answer was almost as popular as the correct answer. 😎
Yup, that's the one I selected and as soon as I submitted my answer, I had a feeling it was going to be wrong.
Here is a quick of the history of sysobjects or sys.objects on the differant server versions.
SQL 2000 - dbo.sysobjects was a System table. Contains one row for each object (constraint, default, log, rule, stored procedure, and so on) created within a database. In tempdb only, this table includes a row for each temporary object. Some did call it the base system table since it was the first object listed.
SQL 2005 - sys.objects system view was created as part of the object catalog. dbo.sysobjects and sys.sysobjects are made as compatibility views. M$ recomends switching over from one to the other.
SQL 2008 - Nothing changes but the server version.
It is important to mention that not only do sysobjects and sys.objects have a differant name, they have a differant schema also. The column names are differant, along with some other things.