I don't know the rules . . . but will just give a few "impressions" is all. Lesa obviously has some experience and it looks positive.
You haven't told us what type of cedar this is. It wouldn't make too much difference to me but what are known as cedars in North America come in both Thuja this-or-that or Juniperus. My limited experience is with the Thuja occidentalis that are often planted in yards and with Thuja plicata, the Western Red Cedar that grows here in our forests.
Just as a observation, I've seen the Western Red Cedar brought down from their cool, wet, often north-facing locations in the mountains and planted in yards. Thirty years later, they are still there growing in the full sun! What I think may be happening is that this cedar is "forced" to live where it does naturally because other trees out-compete it elsewhere! That suggests the opposite of what you are wondering about. The tree, even tho' it often grows in dense cedar thickets and may have some hemlock and other evergreens about, cannot handle too much competition from other plants. . . . maybe

.
We might think of cedar as having some allelopathic properties like the black walnut because its needles, bark and wood are so fragrant. This may not be so. After all, cedar bark is a common mulch. It may suppress weeds (usually with landscape cloth under it) but I've never heard that the bark interferes with the growth of the ornamentals that it protects from weeds.
My limited experience with growing other plants around living cedars has to do with Thuja occidentalis, or what I usually call arborvitae. There was a row of them in my backyard at another home and the neighbor
has had a row on the other side of the fence from my cutting garden.
They do suck a lot of moisture out of the ground. I had to provide sufficient soil moisture to grow the green beans I used to plant beside the arborvitae in my backyard.
They compete with the annual flowers I plant near the neighbor's fence but aren't very successful living in his yard. I can't blame their problems on a single person - there have been a series of renters in that house since the owner turned it over to a management agency, or whatever.
Since running the automatic sprinkler in the backyard made life difficult for his 3 pitbulls, one renter turned off the sprinkler system over 2 summers. The willow and arborvitae began to die.
Things haven't improved with the new neighbor and his 2 pitbulls. To be sure, the willow died and blew down just before he moved in. It crashed into a metal shed which was unfortunate for the arborvitae because a few of them must have relied on the rain runoff from that shed for water. They are the only 5 or 6 arborvitae left alive out of about 15 that once lived on the other side of that fence. Life was tuff for them last year, they may not make it thru 2010.
My guess is that if you take good care of your cedar, Elf, you may be able to grow lots of things around it. Soil moisture would probably be your greatest concern AFTER you have figured out where things can get some sun.
Pecans?
North Carolina State says they "also produce juglone, but in smaller amounts compared to black walnut." NSCU lists several other common landscape trees on that page that you may want to look at.
Combined with the plants listed on that Ohio State webpage that Ridgerunner provides, I wonder whether we really need to worry much about the allelopathy of even the black walnut, if we just do a little planning on what-goes-where.
I hope this helps.
Steve