WhoWho, that's exactly what ds1 used to do when he was younger.
Eg he used to love diggers and dumper trucks and had lots of them, along with a construction site rug/playmat and accessories etc. He'd spend hours setting everything up, putting things in exactly the right place and then no-one was allowed to touch it - sometimes for days. (Which wasn't great as he favoured setting it up in the middle of the living room floor and wouldn't let us move it for days on end.)
Trouble started when ds2 got old enough to want to actually play with the diggers and drive them around on the mat though. 
He even had sticker books, reference books, story books and even dvds about diggers. Actually, he still insists, even now, on having the duvet cover and pillowcase with diggers on that we bought him back then. We only have one set, so I have to get it washed and dried while he's at school because he won't sleep with anything else.
I'll never forget him rendering a local workman speechless when he stopped to point out his digger, complete with it's precise make, model and reference number etc. Iirc, he was barely 3 at the time! 
He has a shelf for all the Lego kits he's built, high above his bed where ds2 can't reach them.
Like Sphil's ds though, there's no room left for any more, but that hasn't stopped him asking for lots of new sets for Christmas. It seems he's currently building up to a new lego fascination. It seems its going to be Lego Alien Conquest - so that will probably end up needing another shelf to house it as well.
Sphil, I saw those air swimmers. The clown fish one is great - but somehow I find the shark one a bit scary. 
Its ds2 that loves Bionicles in this house - ds1 loves the Bionicle computer game, but insists the actual Bionicle figures 'aren't proper Lego'. 