I am also a May 2009 bride. I'm not doing STDs, just skipping right to the invitations. I'm waiting until the end of June/early July to send them out. Airlines post their flights schedules no more than 330 days in advance, so after I see the schedule, I will include that information with my invites and send them out. There are such limited flights to St. Lucia, so that makes it easier on me!
I'd say it all depends on how far along your planning is. If you have a definite place and you know the exact dates, I see nothing wrong with sending out the invitations. If you are a little unsure, then go with STDs.
I will be doing boarding pass invitations, so since I'll be working so hard on them, I didn't want them to become just a formality. I want them to actually invite people. Do what works for you!