Most of the databases I work with are for vendor supplied systems, but it is not uncommon to see some 1:1 relationships between tables, particularly if a single table might contain a large number of columns.

Seems to me that I have read an article from Kimberly Tripp at SQLSkills.com that discusses the performance impact of records > 8k that would cause excessive page splits and might be candidates for separation into 1:1 tables. I'm sorry, other than the general idea of the piece, I can't give you more details. Most of the inhouse DB's I've designed have been for relatively small systems and I've never had to break out columns into 1:1 tables.