Speaking as a QA who has worked on Agile projects for 2.5 years this is a really difficult issue and I still don't have all the answers.
I work as part of a "triplet" (two developers who pair program + one QA) and I am involved in tasking out stories and estimating in planning meetings at the beginning of two week iterations. As adrianh mentioned above it is essential for QAs to get their voice heard in the initial sprint planning. This can be difficult especially if you are working with Developers with very strong personalities however QAs must be assertive in the true sense of the word (i.e. not aggressive or forceful but respectfully seeking to understand the Truth/PO and Developers/technical experts whilst making themselves understood). I advocate producing QA tasks first during planning to encourage a test driven mentality - the QA may have to literally put themselves forward to get this adopted. It is opposite to how many people think software development works but pays dividends for several reasons;
QA is heard and not relegated to being asked "so how are you going to test that?" after Devs have said their piece (waterfall mentality).
It allows QA to propose ideas for testing which at the same time checks the testability of the acceptance criteria while the Truth/PO is present (I did say it is essential for them to be present in the planning meeting didn't I?!) to fill in any gaps in understanding.
It provides the basis for a test driven approach - after the test approach has been enunciated and tasked the Devs can think about how they will produce code to pass those tests.
If steps 1 - 3 are your only TDD activity for the rest of the iteration you are still doing a million times better than the scenario postulated by Steve in the first post; "Developers thrash around trying to accomplish their tasks. Generally the tasks take most of the sprint to complete. QA pesters Dev to release something they can test, Dev finally throws some buggy code out to QA a day or two before the sprint ends and spends the rest of the time fixing bugs that QA is finding"
Needless to say this comes with some caveats for the QA;
They must be prepared to have their ideas for testing challenged by Devs and Truth/PO and to reach a compromise; the "QA police" attitude won't wash in an Agile team.
QA tasks must strike a difficult balance to be neither too detailed nor too generic (tasks can be written on a card to go on a "radiator board" and discussed at daily stand up meetings - they need to be moved from "in progress" to "completed" DURING the iteration).
QAs need to prepare for planning/estimation meetings. Don't expect to be able to just turn up and produce a test approach off the top of your head for unseen user stories! Devs do seem to be able to do this because their tasks are often far more clear cut - e.g. "change x module to interface with z component" or "refactor y method". As a QA you need to be familiar with the functionality being introduced/changed BEFORE planning so that you know the scope of testing and what test design techniques you might apply.
It is almost essential to automate your tests and have these written and "failing" within the first two or three days of an iteration or at least to co-incide with when the Devs have the code ready. You can then run the test/s and see if they pass as expected (proper QA TDD). This is how you avoid a mini waterfall at the end of iterations. You should really demo the test to the Devs before or as they start coding so they know what to aim for.
I say 4 is "almost essential" because the same can sometimes be successfully achieved with manual checklists (dare I say scripts!) of expected behaviour - the key is to share this with Devs ahead of time; keep talking to them!
With regards to point 2 above on the subject of the tasks, I have tried creating tasks as granular as 1/2 hour to 2 hours in size each corresponding to a demonstrable piece of work e.g. "Add checks for incorrect password to auto test - 2 hrs". While this helps me organise my work it has been criticised by other team members for being too detailed and has the effect at stand ups of me either moving multiple tasks across to complete from the day before or not being able to move any tasks at all because I have not got onto them yet. People really want to see a sense of steady progress at daily stand ups so it is more helpful to create tasks in 1/2 day or 1 day blocks (but you might keep your own list of "micro-tasks" to do towards to completion of the bigger tasks that you use for COMMUNICATING overall progress at the stand-up).
With regards to points 4 and 5 above; the automated tests or manual checklists you prepare early should really cover just the happy paths or key acceptance criteria. Once these pass you can have planned an additional task for a final round of "Exploratory testing" towards the end of the iteration to check the edge cases. What the Devs do during that time is problematic because as far as they are concerned they are "code complete" unless and until you find a bug. Some Agile practitioners advocate going for the edge cases first although this can also be problematic because if you run out of time you may not have assured that the acceptance criteria have been delivered. This is one of those finely balanced decisions that depends on the context of the user story and your experience as a QA!
As I said at the beginning I still don't have all the answers but hope the above provide some pointers born out of hard experience!
