I'm a bit late to this thread but have been wondering about those cheap wall-plug Chinese grid-tied inverters as well. I just watched Dave's video about the paltry 6c/kWh feed-in tariff he gets, and the thought crossed my mind that by feeding in the power through the wall, you'd be effectively increasing your feed-in tariff to equal what you pay for electricity, assuming you have an electricity meter that can run backwards in the event you generate more power than you're using at the time.
However some research on this showed that here in Australia, it seems that it is not permitted to wire up grid-tied power sources in this way. It seems like you need two separate meters. I found a story of someone who had their professional solar system installed this way and their meter was running backwards, and their energy supplier's billing system could not cope with a negative difference in meter readings and consequently they ended up with a massive bill, which took some sorting out and ended up with them having to get the system rewired properly.
So it sounds like if you were to use such a system in Australia, you'd have to be careful to make sure that overall you used more power than you generated so that each meter reading showed some increase from the last, otherwise you might get unwanted attention from the electricity retailer trying to work out why you aren't using a feed-in tariff like you're supposed to.
I can't find anything about the legality of the units themselves, but I wouldn't be surprised if it's the same as unapproved POTS telephone equipment - it may not illegal to buy or possess, but it is to connect it up to the network.
EDIT: Found
this explanation which includes some info from an SA Power Networks document:
Customers are also reminded that they should not connect an SEG [Small Embedded Generator] to a mechanical disk type meter. This will cause the meter to turn backwards giving the appearance that a customer has consumed less energy from the grid than they actually have. This is considered to be a breach of section 85(1)(b) of the Electricity Act of 1996, as the exporting of energy interferes with the meter's ability to correctly measure the consumption of energy supplied and used by the installation connected to the Network. Civil penalties up to a maximum of $10,000 or two years imprisonment can be applied in these situations.
That page also explains that modern meters (those with digital readouts) will count exported power the same as imported power, so using 1 kWh and feeding back 1 kWh will show as if you've used 2 kWh total, so you'll be billed for both the power you've used *and* the power you've exported. So if you have a digital electricity meter, one of these plug-in inverters will cost you more than not using it.