Which transaction? Is adding trust a "transaction"?
Yes, rather than "transaction" I should have said "ledger entry".
Actually "transaction" is the right term. To change a ledger entry (such as adding/removing/changing trust) requires a signed transaction just as payment requires a signed transaction. They're just different types of transaction.
The real reason I posted: since this topic was started a Ripple blog post tries to cover this subject. They gloss over a few things but it's a nice overview.