For the sake of this post, let's just assume that testers and developers are just one big happy family with the shared goal of shipping working software. On rereading this post it's definitely preachy, but I've been burned in the past by not being inclusive of the testers and my current client definitely suffers a bit from a too developer-centric viewpoint.
Testers are pigs in the "people who have a direct stake in the project's success" manner. Sometimes we fall into a trap of thinking that productivity and schedule constraints are strictly related to developer time, but the testers are just as important. I've got a case at work where 2-3 days of developer time probably translates into a couple weeks of testing. Asking me alone when we can deploy isn't that useful because the testing is the bottleneck. The tester's time is paramount. For that reason, we need to get the testers (the tester in reality) every chance to be forearmed with prior knowledge of the work being done. Planning has to include the question "how long do you think you need to test this?" Is it really fair to dump a feature on the testers that they've never heard of and say "test this by the end of day tomorrow?" Do you like it when somebody drops a complex architecture with a design pattern you've never seen on your desk and says code this by next week?
Do unto testers as you'd have done unto you.
Remember to involve the testers anytime you're launching a new project, scheduling a production rollout, changing requirements, or really doing anything that alters the course of the project. A lot of the time we implicitly assume that the testing time is linearly dependent upon the development time, but that's not always true. The testers really need to have a say in the project scheduling, both to ensure that they have adequate time to do their job, and also just to know what it is that they're going to be expected to test. In an ideal Agile team the testers are fully involved with each user story in near lock step wth the developers, so it's easier to keep the testers into the normal flow of a project. Ignore the testers in your planning, and you pay for it later.