This is one of those questions that arose out of my ‘e-mail grab-bag.’ The ‘ampersand’ is not improper or slang; it has basically just fallen out of favor over the years. It would never be used in the body of a letter of text. But it is used frequently for say departments i.e., "Community Development & Housing" etc.; and when addressing couples such as "Mr. & Mrs. Smith"
I found a wonderful site on Wikipedia that addresses the history of the ‘ampersand’:
The ‘ampersand’ appears to be a very ancient sign, originating in Roman culture. The ‘ampersand’ symbol has been found on ancient Roman sources dating to the first century A.D. Marcus Tullius Tiro, Cicero’s secretary of 36 years, is credited as its inventor.[As for it’s usage, here is a section from Wikepedia:
Although common in handwriting before typewriters came into widespread use, the ampersand has lost popularity in recent years, and it has become standard in most contexts to write out the word "and." It is still though, considered to be an acceptable alternative to the word "and."
The main surviving use of the ampersand is in the formal names of businesses (especially firms and partnerships, particularly law firms, architectural firms, and stockbroker firms (the names of these also nearly always omit the serial comma)). A common explanation as to why the plus sign is not used instead is that a partnership is a relationship, and therefore more than simply adding one person with another.
The ampersand is also often used when addressing an envelope to a couple: "Mr. & Mrs. Jones," or "John & Mary."
The ampersand is also used for titles, such as Harry & Tonto, as well, and in some other proper names. In these cases, & is interchangeable with the word and; the distinction between them is mostly aesthetic. However, in film credits for story, screenplay, etc., & indicates a closer collaboration than and; in screenplays, for example, two authors joined with & collaborated on the script, while two authors joined with and wrote the script at different times and may not have consulted each other at all. 
Hunter S. Thompson used the ampersand instead of writing the word "and."
In APA style the ampersand is used when citing sources in text such as (Jones & Jones, 2005).
The phrase et cetera ("and so forth"), usually written as ‘etc.’ can be abbreviated &c. This is because the ampersand originally stood for the Latin et.
Note however the Hebrew Keter (Crown) and Egyptian Ra and see the note in Sin / Shin on Egyptian royal divine solar Uraeus.
Ampersand may read in German am and French Persan[d] meaning in Persian and appears to be a secret code in certain old coded texts that the text is to be read either literally in Persian or, more commonly, reading R to L. Note that when doing so, numbers do not change as the Arabic system is as our own.
look for more information at the source URL: http://en.wikipedia.org/wiki/Ampersand