Looks to me that 0 is the only really non-premium number but it is not very bad. 4 can mean "for" which balances the Chinese problems, somewhat. 1 and 2 are probably the best numbers. Wonder if the similarity to sex is the issue for six?
I noticed in the recent 3 character .net extinction that the lower numbers went first, more or less in order, zero last. There were some great letters that were left when all the 1, 2, and 3's were gone.
I am starting to wonder if 3 and 4 character domains will soon be pressured by a lot of end users owning mobile websites. If length is such a limitation of those devices, and if (the big if) that length issue continues, websites serving the mobile market will want to be less than the length of the browser space - 10 characters, so I have heard. Giving one space left open, so the user knows he is seeing the entire URL, and counting the dot, that means domains of five characters or less for .com and net, four or less for .info and .mobi, and 6 or less for .us, .eu and other country code domains.