I have to say that I really fancy having a small one installed in the garden. Obviously not something with a 2km diameter crater cos a) the garden isn't that big and b) that would be showing off.
Say perhaps something about the size of a mole hill or perhaps, if you have the space, up to the size of a good ant hill.
On the plus side it is really going to shut up the neighbours who have been prattling on about their water feature and how soothing the sound of running water is in the evening as they sit out with a glass of Pinot Grigio and a selection of Tesco Finest nibbles.
Watch then shut up when you bring them round and show them "Olympus Mini" that you just picked up from B&Q. It will sit there rumbling malevolently and occasionally throwing up a shower of molten pebbles. Tell them that it is set on 3 and the dial goes up to 11 !
I'm guessing that the smoke and ash will be a downside but it cant be any worse that a garden bonfire so don't let that put you off this idea.
If you think about it - it could be really handy if you need any paving done - simply set up some asbestos shuttering and let the lava flow - instant Deccan Traps in miniature and you get the path laid in moments. You might want to put up some sort of barrier to keep the dog/cat off it for a coupel of weeks until it cools down as I'm pretty sure that molten lava path and paws are not a good combination. Mind you there could be a side line in thermal boots for pets.
Any vulcanologists with a side interest in DIY will get this. Any other readers will be reaching for the Google toolbar by now.
I really think that Dermot Gavin should add this to his repertoire - it could help reinvigorate his garden design career.