For discussion purposes, here's some quotes I got from a gOOgle search; I don't necessarily agree with them acting like all experienced frequent travellers know this. *I personally* never tip when I ask the concierge for a reservation in order to avoid the hotel's $3 charge for a phone call (my pet peeve). But if I do it repeatedly, I leave him/her something at the end of my stay. And tickets or a bus trip would draw a bigger tip, which I never forget. In any case, all these sources seem to think I'm a bit of a cheapster, as I just don't hand the guy $3-10 for calling around the corner to a restaurant, when I don't speak the local language; I've paid the hotel plenty to start with, and in many cases they've tacked on a "service" charge already (outside the US). I'll tip for extras, but the effort of making a phone call isn't an extra. Get me in somewhere exceptional, that's an extra. Arrange a special tour, that's an extra. Send a package for me, that's an extra.
From Fodors.com:
You should also tip your hotel concierge for services rendered; the size of the tip depends on the difficulty of your request, as well as the quality of the concierge's work. For an ordinary dinner reservation or tour arrangements, $3-$5 should do; if the concierge scores seats at a popular restaurant or show or performs unusual services (getting your laptop repaired, finding a good allergist, etc.), $10 or more is appropriate.
From FineTuning.com:
Webster’s Dictionary defines ‘concierge’ as French meaning ‘to serve.’ Concierge services vary from small jobs to more complicated ones. A concierge can make arrangements for your social functions or find that perfect gift for a special someone. If you’re a busy professional, concierge services are imperative. It isn’t necessary to tip, however, depending on the extent of the services, a $5 to $20 tip will be appreciated.
Most upscale hotels have concierge services. It isn’t necessary to tip the concierge unless he provides a service to you. If so, you’ll want to have some small bills on hand to tip him once the service is performed. A $5-$10 tip with a word of appreciation is sufficient.
From HowStuffWorks.com:
The standard amount for the concierge is $5 to $10 depending on how helpful he or she was. If the concierge suggested and made reservations for you at a great restaurant, then you might tip more.
From CNNMoney:
$5 for getting you tickets or reservations ($10-plus if they're hard to get). No tip required when you ask for directions
From DrDaveAndDee.com:
Concierge: nothing for simple questions. But, if they make restaurant reservations, obtain theater tickets for you, make travel arrangements, then $5-$10 per task performed. Put the gratuity in an envelope with a note of thanks and give to the concierge.
From iParenting.com:
Concierge – $3 or more for tickets; if it took extra time, like getting tickets to the best Broadway show, the tip should be 10 percent of the cost of the tickets
From MaxCConcierge - $5-10 for help with hard-to-get dinner reservations or theater tickets. Tipping is optional for just plain advice. Tipping can be done at the end of the trip or at the time of service, just keep is straight so that you are fair.
From CNN (again):
More upscale hotels typically have a concierge who assists travelers during their stay, but some people don't realize they accept tips, and others "wildly overtip," Walther says. The National Concierge Association recommends tipping anywhere from $5 to $20, more when the concierge does something extraordinary.
If you're just asking for a recommendation on where to eat lunch, a tip probably isn't necessary, Walther says, but if the concierge gets you theater tickets and a table at a top restaurant after the show, $20 to $30 might not be inappropriate.
Walter Sanders, of Diners Club International, agrees.
"If a concierge could get me tickets to 'The Producers,' I would consider that miraculous. You're talking about a very generous tip there," he says.