Do tubeless tires offer significant benefits for a cyclocross bike?
So I decided to take the plunge and try the tubeless wheels.  I've got about 80 miles of road and gravel riding on them now, and figured I should start formulating an answer.  I ended up using the stan's alpha 400 rim because of my weight and the lack of rim brake compatibility in their cross specific rim.

So far it's been as advertised.  The tires can run at much lower pressures than I was able to run with tubes, to the point that if I come back onto the road I actually feel like the tire is going to tear off the rim when taking tight corners too fast (these are corners on neighborhood trails and such, it is not nearly as pronounced when riding on real roads).  The lower pressure lets the tires hook up really well off road - probably not as well as a wide set of tubulars but much better than the tubed tires I've used in the past.  So I would say yes, there are significant benefits.

There are a few gotchas to be aware of though, namely the lack of tire selection - the tires I wanted to run would not lock into the bead.  Even at low pressures they would blow off.  I ended up going with a set of (non-tubeless) continentals that my shop had lying around.  It took a while for sealant to permeate the tire, but they are working great now.  I think there are only a handful of tubeless specific cross tires at this time, but feel certain more will be coming.  With the alpha (road wheels) you also need a special rim strip (in addition to the yellow tape) for higher volume cross tires.  With some tires this is not needed (or even detrimental), but it ended up being needed for mine.  I'm sure you could jury-rig something but I would just plan on spending another $40 or so on these strips (just a rubber strip with a valve stem built in).  I've been told this is not needed with the iron cross rims.

All things considered, I think it was worth the hassle so far.  The stans wheels are great - very stiff when cornering, and I saved almost a pound at the rims off what my factory wheels weighed.  I will come back and post more once I've done some singletrack riding and a race or two.

edit - so I've done some singletrack riding and one race, and now I'm really starting to see the benefit.  The stans wheels don't really allow super low pressures (I burped at about 22 psi while preriding the race course) but they feel great at 30.  This is for a 220 lb rider with 35mm tires, using the rubber strips.  The wheels hooked up really well on the slick grass course, and held up well to dismounts/remounts.  At this point I'm ready to call the experiment a success.  Looking back the only thing I would have done differently is go with a tubeless-specific tire (it took a long time for sealant to sufficiently permeate the non-tubeless tires I used).