Learn English – “Sign up” vs. “signup”

As for writing ‘Web site’ as one word, it is true that this is seen a great deal, but then, so is the spelling ‘recieve’ which is just plain wrong!

They do not actually provide an argument against website as one word. The fact that orthographical errors exist at all is not exactly compelling evidence. Consider this evidence:

1. Obviously, similar compound words exist, both in terms of use and official definition: streetcar, doorman, jailhouse. There are hundreds of these.

2. As VonC mentioned, both "website" and "Web site" are offered in most dictionaries.

3. Regarding capitalization, many words derived from proper nouns do not retain capitalization, e.g. narcissist, sodomy, atlas, echo, siren. This was touched on previously regarding days of the week.

I think it is safe to say that website is clearly used hands-down more often than Web site, so I think it should certainly be regarded as at least a valid variant. (And it is, by most authorities on standard English.)

In the case of "acronyms" such as R&D the spaces would normally be omitted, but where the surrounding elements are words (for example, Tate & Lyle), spaces are invariably present.

Here's a link to Marks and Spencer's small print, where they refer to themselves as both M&S and Marks & Spencer on the same web page.

Just to clarify a point arising in comments elsewhere, in the title of a guide to HTML & XHTML the spaces are expected - although both surrounding elements are acronyms in themselves, they do not form a new single acronym when conjoined with an ampersand.

Also note that although at least some style guides (incl. Chicago Manual of Style) explicitly rule against spaces in acronyms, as @nohat points out, they are only style guides - there is no absolute rule in play unless your commissioning editor requires adherence to such. For example, in the UK, hospital Accident and Emergency departments are invariably A & E (with spaces).