I'm just wondering how you deal with the above situation. I have a forms
library, which suffers from this if a form is open when a workflow trys to act
on the item. The document is open but isn't checked out as such as the 'Checked
Out To' field is blank & the file doesn't actually show up as checked out. I
generally put a 5 min pause in the workflow to deal with this, but there must be
a more graceful way to deal with this.

Also,

Does anyone know of any good resources/books that take you through how to write
custom SPD activities?