I have had this happen a couple of times as well. Looking at my pending reservation emails, the rate is on the email, but I have seen some where they are in a link. I usually forward my confirmation emails to Tripit. I have started taking screen shots of the rates and putting those in Tripit as well since I have had a few reservations where the rate 'disappears'.

I don't know why sometimes it does and sometimes it doesn't, but its a great point by the OP to remind us to take screenshots if the rate isn't specified in the body of the email.