I propose special tag, something like [convert][/convert]

Examples of possible usage:
[convert src=USD dst=BTC site=mtgox.com]6[/convert] should show ~0.9 using mtgox.com to determine rate

if some attribute is not set, it should use defaults:
dst - BTC (it will allow convert from any currency to BTC - [convert src=EUR]15[/convert])
src - USD (it is shortcut for most famous international currency [convert]20[/convert])
site - mtgox.com (site with largest volume)

Gribble, the IRC bot on #bitcoin-otc, already has a nice syntax for doing this and more.