I don't think that saying that you can't compete at all and that it is ungracious for other teams to attend two competitions is valid. You still can compete, just at another event. My older brother's team registered late in 2005 and had to go all the way to Canada to compete along with another team from West Virginia who also registered late. Events fill up fast if you were unable to register in a decent time that is your fault, to put it bluntly. Also, other teams going to two events it not ungracious. Teams attend multiple events to get more experience and learn new things and meet new teams. That is one of the reasons that the Michigan districts were so popular as teams got the experience/fun in having more matches.
Staying on the wait list is a good idea as spots may open up as other teams might drop out for some reason or another, but I would highly suggest that you find another regional before events closer to you fill up. And if for some reason you have to travel to compete this year, who knows, you could have a lot a fun with it!
