I have read Implosion doesn't work against undeads on this server because it is considered a fortitude save (?) and undeads are immune to spells based on fortitude save.
My face been: 
Following basic laws of physics:
http://en.wikipedia.org/wiki/Implosion_%28mechanical_process%29
Why material beings (flesh or bones) can't collapse on themselves?
To make it quick, I think saying they can't implode is the exact thing to say they can't explode. 
Okay firstly, the plural of undead is still undead. Whether you have one or one billion, you still use the word "undead". Yes, I know, english is a weird language.
Secondly, don't quote physics when arguing how magic works in a fantasy game. It's irrelevant.
Thirdly, here's the D&D rules for undead creatures:
http://www.d20srd.org/srd/typesSubtypes.htm#undeadTypeImmunity to any effect that requires a Fortitude save (unless the effect also works on objects or is harmless).
and here's the rules for the
Implosion spell:
http://www.d20srd.org/srd/spells/implosion.htmImplosion
Evocation
Level: Clr 9, Destruction 9
Components: V, S
Casting Time: 1 standard action
Range: Close (25 ft. + 5 ft./2 levels)
Targets: One corporeal creature/round
Duration: Concentration (up to 4 rounds)
Saving Throw: Fortitude negates
Spell Resistance: Yes
You create a destructive resonance in a corporeal creature’s body. For each round you concentrate, you cause one creature to collapse in on itself, killing it. (This effect, being instantaneous, cannot be dispelled.)
You can target a particular creature only once with each casting of the spell.
Implosion has no effect on creatures in gaseous form or on incorporeal creatures.
The spell ONLY works on creatures, and a Fortitude save negates any damage. Undead are immune to effects that require Fortitude saves, thus undead are immune to the
Implosion spell. Period.
