Form what I understand so far, I don't think could currently make something called Arduinoplugins® without permission, but you could easily do "Plugduino" or something like that. The schematics are open and you can make your own PCB's. The things it seems you can't currently do are: use the name to make microcontroller boards that are similar to the Arudino Diecimila, BT, etc in name, or in trade-dress, i.e. "look".
But the schematics are released CC, so I think you could make something like "Superboard!", and make it blue too, and say it was Arduino compatible, as long as you made the language clear enough that there was no confusion between SuperBoard! and Arduino.
Anyway, I stick with the idea that the name should be licensable for free, as Linux does, and that there should be no commercial or other restrictions on what you can do with the name and reference designs. The Linux industry is worth what, billions? If Arduino has sold 12,000 board in two years, at say $20 average gross per board, that makes $120,000 a year for the manufacturers and distributors to split up. That's not a whole lot of cash for a big company... it's totally a small business/ home business amount. Once you split that between four or five manufacturers and distributors, it will be 20K each a year, meaning it will be very hard for anyone to dominate the supply. Even better, it will mean that there's price competition, as people will want to increase their piece of the pie.
There's no price competition now... the USB board debuted last year at Sparkfun for 29.95, and the Diecimila now sells for $34.95, about a 16% increase. Given that manufacturing costs would be about $10 or so in 1000+ quantities, there's a whole lot of room for the price to go down! So instead of Sparkfun making its $60,000 a year profit, or whatever it's making on the Arduino, that might get split up between several manufacturers, who would compete and drive prices down. Unless they formed a monopoly! LOL.
Lots of interesting issues.