Is it ever appropriate to use a space before and after an ampersand?

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).

No spaces is most often used, and supported by style guides, except in special cases. For those who have access to CMOS online, here's a link to the section advocating this:

10.10: No space is left on either side of an ampersand used within an initialism. R&D. Texas A&M.

For those who don't, search the CMOS site for "ampersand initialism" to catch a glimpse of the rule. If the Thesaurus.com style guide is worth anything, it has the same idea.

Yes, "acronyms"/initialisms have no space. When the ampersand is used inline the spaces are present.

Bear in mind, the ampersand is a highly stylised rendering of the Latin "et" which means "and" (perhaps a scribal shortening). I have no doubt that once it was pronounced "et" rather than current "and". So inline it is a word, in acronyms, a symbol.