"Uniform Resource Locators (URL): Generic Syntax and
Semantics"
ftp://ietf.org/internet-drafts/draft-fielding-url-syntax-09.txt
has a discussion of this. I think that you will find it is a
pretty good reflection of current usage including conversion
functions.
It says the the URL: in <URL: ...> is unnecessary, and that any
of <..>, "...", or simply guaranteed whitespace should be used to
set an URL off from surrounding non-URL text.
-- Al Gilman