Well, writers block sounds rather grandiose. But that's basically what it is. I think of things to write at the most inopportune times. Like when I'm driving, or in the shower. Then, when I sit down to write them...nothing.
I stumbled across this blog entry. How to concentrate on writing. Usually these lists seem pretty silly to me but I find this particularly fitting. I'm often writing for work. Either proposals or customer documentation. Staying on task is quite difficult at times and for the reasons Matthew lists here. I think I'll tuck this into my toolbox for the next time I've got to get through a writing assignment.