It varies by airline, and I'm not sure what United's specific policy is, but for both US Airways and Delta you must have either a boarding pass, paper ticket, or E-Ticket receipt (either on travel agency/airline paper or e-mail/confirmation screen print-out) to get past security. So if you have one of the latter two and have only carry-ons, you should be able to go through security and check-in at the gate or club. And the gate lines will be very short with almost everyone checking in at the counters, so you won't have much of a wait (and agents will have more time to help with waitlists). Two disclaimers: (1) Policy I just stated is specific to Delta and US Airways; United Policy may differ; and (2) [and most importantly...] sometimes airline agents (this case security agents) aren't familiar with the by-the-book or common-acceptance policy, and might not let you through without a boarding pass, even if company policy says etkt receipt is okay.