I bought a new Antec P150 Case from overclock.co.uk last week, and have had the same old problem as everyone else. The serial number is S0509...
I have been in contact with Antec, and they say that there is an incompatibility with Asus A8N and P5 motherboards but not with any other models. Apparently the problem with other mobos arises from the fact that the PSU is ATX12V 2.2 compliant. The PSU leterature does say it is backwards compatible with the ATX v2.01 standard, but, one is ATX and one is ATX12V, so older boards that are ATX V2.01 compatible arent guaranteed to work.
Basically, the switchoff problem occurs as the minimum current on a 12V rail for ATX12V is 1 amp, and if you try and draw less than an amp the rail wont turn on - it just powers off as soon as you turn the machine on, or when the rail draws less than an amp for 'a while'. Obviously newer mobos and graphics cards that require extra power wont have this problem. Antec support told me to load up as much hardware in the box as possible and turn the TriCool fan on full - I did this; when I have 2 disks and the fan at the highest setting the machine boots fine!!
Obviously running the fan on full isn't what you want to do unless you really need to get rid of some serious heat. The reason I bought this case was because my last one sounded like a jet engine and I was fed up with the noise, so the solution is to load as many devices on the same 12V rail (there are 3 of them coming off the PSU) as possible so you dont have to run the fan hard. I cant find out which sockets use which rails, but when I do im going to make sure that the rails that I use are drawing over an amp as this will probably solve the problem.
If I cant get it to boot after adding an extra disk with the fan on low, im going to try something stupid like getting hold of a 47ohm resistor rated at 10W or more and hang it out of the back of my case attached to the 12V rail. This will draw another quarter of an amp or so and should dissipate the generated heat okay, although I might add a heatsink or try and sink the heat though the case. Of course this is a completely stupid idea and I wouldnt recommend anyone do it, but it'll probably work.
One more thing; when my PC does boot, if I try and shut it down it wont power itself off and I have to do it manually, I notice that some other people have the same problem sometimes. I dont know how to get around this but I asked Antec support. I suspect its something to do with the low current draw, but who knows. Hopefully they will get back to me with an answer.
Oh yeah, and warranty sticker broken on arrival