hi peter,
since you already took it apart, i think you don't need the [guide|1117|guide]
you already tried a few things, but i tell you what i would do in that case -s o if there is anything mentioned what you already did - ignore it ;-)
does the led flash a few times when you power it on ?
try to start from the setup dvd that came with the mini(press c when you switch on the mini)- (btw - what os do you use ?)
if it starts from the disk, try the disk utility to check/repair the permissions/harddisk
if that wont work - go the hard way
remove almost everything whats inside - ram, wifi card, hdd & dvd
try to power it on - normally the led should flash 2 times in that case - the mini would recognize that no ram is installed
normally there are beep codes - but i had a mini that didn't make any sound
1 beep = no RAM installed
2 beeps = incompatible RAM types
3 beeps = no good banks
4 beeps = no good boot images in the boot ROM (and/or bad sys config block)
5 beeps = processor is not usable
the flashing led is beep code plus 1 (no ram equals 2 flashes)
if that works, shut it down and try putting the ram in and start it again - if that also works, you should see a grey screen with a folder/question mark - shut the mini down
now connect the hdd and try booting the mini again - if it works - you should be able to start the system
now you can try the same thing with the optical drive and the wifi card - if the system stops on any point - you have found the faulty part.
you can also change the order or better - if the hdd won't work, try to go ahead with the optical drive and so on.
sometimes is the disconnecting/connecting game the only way to rule something out.
but keep in mind - the psu can be faulty
i hope that i didn't forgot anything ;-)