It can all be traced back to a remark by Robert Bringhurst in his influential book "The Elements of Typographic Style" in which he suggests that in titles it is worth using the most decorative & sign we have (some fonts have different variants available) or even use the italic ampersand in roman context, because the italic & is often more decorative (ultimately they all come from a stylized "et").

I think many have taken it too far and the results sometimes look pretentious. An example still on the safe side (IMHO) is in pg's book:

http://t3.gstatic.com/images?q=tbn:ANd9GcR2J5Pw_xUwVNxV_z504...

And the typograhy book (highly recommended):

Non-affiliate: http://www.amazon.com/Elements-Typographic-Style-Robert-Brin...

Affiliate: http://www.amazon.com/exec/obidos/ASIN/0881792063/mybookbox-...

Coincidentally I have just started reading ETS. Care to tell me where he actually talks about ampersands in the book?

The ampersand is a subject of major fascination between type designers, and through that - the wider masses of graphic designers. See, for example - http://www.typophile.com/node/66611

Ampersand to the designers is what sort() is to programmers :)