The concept of "hard calendaring" only events that occur at a specific date/time as David talks about is something I find very helpful and rigorously follow, with one exception:
As an attorney, I have to deal with what I'll call legal deadlines which must be met or very bad things happen. These are day, but not time, specific deadlines imposed by court orders and rules. For example, a Notice of Appeal must be filed by a certain day, but it can done anytime that day, as opposed to a meeting or appointment which has both a date and time associated with it.
These are quite distinct from the usual types of business/personal deadlines that are discussed in the GTD book. The legal repercussions that flow from missing one are why I don't feel comfortable handling them as I would any other normal "Next Action" for fear that they will get lost in the NA forest and inadvertently missed.
At the same time, I don't really want to clutter up my calendar by putting all legal deadlines on the hard calendar with some fictional time associated with them (for example, "File Notice of Appeal, 2-10-2005 @ 5 p.m.), when all that has to happen is that the action simply occur sometime BY or ON that day, not some specified time. That said, I don't rule out the hard calendar as the best place for these types of items if someone can make a convincing case that they deserve an exception!
I rely TimeMatters for my project/case management, which is wonderfully flexible in separating hard calendared items v. Next Action/to-do items and linking everything to a case/project.
If any other GTD-attorneys on the list have grappled with this question, I would appreciate hearing how you handle these types of "legal deadlines" in your system, regardless of what software/paper system you use.