To provide some real-world examples:
My Projects list currently contains 6 items, all of which I plan/hope to finish this week. I have 10 Next Actions (some of them came from Tickler reminders, and aren't full-scale Projects, such as "Change all clocks in house to atomic time").
I currently have 164 items in my Someday/Maybe list.
Last night, I got inspired by some videos of the TED conference, and thought up a topic for a series of blog posts. I put that new idea on my Someday/Maybe list, not on my list of Projects. I have other active Projects that I'm working on right now. I'll get back to that idea on Sunday during my Weekly Review, when I can decide if it's worth working on, but I'm not going to compromise my current work load.
I've solved the problem described by sorting the items in one list: creative projects go at the top of the list, and non-creative ones at the bottom. If I'm feeling creative, I know to look near the top of the list. But--and I think this is important--they're all in the same list, so I'm evaluating them all at once.