I have been working seriously with the Pomodoro technique recently. I find it genuinely useful for achieving focus on a single extensive task. I used it to plow my way through an extensive e-learning trajectory (3 hours of material) and to focus on writing documents.
Engaging with the Pomodoro technique made me realise that GTD offers relatively little in the way of strategies for executing, so Pomodoro fits nicely into the DO layer of GTD.
If you want to integrate GTD with pomodoro all you need to do is use the work inventory aspect of GTD, look carefully at your Next Actions and select any that you want to move on that need a substantial effort (30+ minutes). These you can block into your diary as pomodoros. I tend to label tasks as "Review course training material 2PD" which means that I should block out 2 pomodoros worth. I do use a pomodoro sheet to record my progress on pomodoro tasks, but I process the "urgent and unplanned" part of the sheet back into GTD.
An unexpected and not totally welcome effect of Pomodoro-ing is that you suddenly realize quite how little focussed intensive effort you manage in one day. My current record is six Pomodoros, though it should be said that I am currently only 60% available as I am recovering from an eye operation. The technique also makes it very visible when you under or overestimate the time needed for a task.
If you have some kind of standard block of intensive work (perhaps a regular report to write) I recommend blocking it out in Pomodoros and seeing how your estimate of the time needed matches up to reality.
Where GTD conflicts a little with Pomodoro is the handling of interruptions. In GTD the emphasis is on flexibiltity: you snap round, handle the interruption and then return to the inventory of your work, perhaps with a different focus as a result of the interruption. Pomodoro emphasises remaining focussed on the task at hand, straight-arming incoming interruptions to handle after the pomodoro has expired. Both have their advantages. As I gain more experience mixing the techniques I will post further thoughts.