It takes a determined search, but paragraphs 71-78 of Schedule 27
of the Civil Partnership Act 2004 - helpfully headed "MINOR AND CONSEQUENTIAL AMENDMENTS: GENERAL" contain several amendments to the British Nationality Act 1981.
I haven't done a line-by-line comparison, but the basic thrust is that wherever "marriage" is mentioned, it is now accompanied by "or civil partnership"
The URL is
http://www.opsi.gov.uk/acts/acts2004/40033-bb.htm - and be prepared to do a lot of scrolling...
The most significant changes for people here are probably that people in a civil partnership with a British citizen ---
---will have their pre-naturalisation UK residence requirement reduced to three years (from five), and
---they will only need to have ILR
on the day of application for naturalisation, rather than for the 12 months leading up to that day, and
---they will not need to intend to continue to have the UK as their home or principal home.
I haven't searched for the Order that brings the Civil Partnership Act into force, but the Home Office website says that it is wef 5 December 2005 - despite which they do not seem to have published anything substantive on the nationality aspects of the 2004 Act yet.
paul