Even at current difficulty, one of these things will earn about 0.006 btc 0.15 btc per week. It won't be long before it will take months before a 300MHps device earns enough to make a reasonably sized transaction. Otherwise there'll just be more SatoshiDice-like tx in the blockchain, and you'll have to pay larger fee when you want to spend the coins you mined with it.
So you'll need a bunch of them, and this means, as Kano suggested, messy USB hubs and this really works against the basic idea of a simple little plug in device.
I really do like the idea, but it needs to provide an order of magnitude greater hashrate to make it useful for mining - like the larger boards. If you just want to use it to support the network with this 300Mhps model, fine. But if you want to earn btc with it, it's not quite there yet.
Seems we're on the same page.
I hope ASICMINER sells both. I'd buy some of the minis just to have something to cart around and show people. And I'd buy a monster (if I could afford it - or a share in a monster otherwise) because I'm not just a believer, I'm a true believer.
In terms of making money for shareholders, I think the minis would be more potentially profitable - the novelty value of a pocket-sized asic bitcoin miner is hard to guess at, but I put it at a good size.
And as for the "dust", yes, if you mine with a pool and set to 1m
BTC payouts then (assuming your pool will even do that) then you'll have to hang on to those transactions for a _long_ time before you can afford to spend them without getting screwed in transaction fees. Personally I have a 1
BTC payout limit on my pool. But again, you aren't mining with a mini to earn coins - you're mining with a mini because you believe in Bitcoin and want to keep the hashing power distributed.