Some guys on the THW list are working on "porting" 40k races into the 5150 system.  So far, the Orks are getting the most attention, since they're among the most colorful of the nonhuman races.

While the initial efforts were working on adapting their mob mentality into a sort of "hive mind" rule (like the 5150 Hishen), I counter proposed something that is less complex but still gets the idea across.  After a few rounds of refinement, here is the core Ork Boyz "Waaaagh!" rule:

Waaaagh! - Any group of 10+ Boyz in a group benefit by gaining an extra Leadership dice on any reactions that allow leadership dice.

Simple and to the point, and means that a large group of orks can practically lead itself, and becomes very determined when there is an actual leader "Nob" along for the rumble.

I've also made the Orks "Tuff":

Tuff - Orkz are very hardy and can survive wounds that would destroy other races.  An Ork is never OOF, but always counts an OOF result as Knocked Down.  This includes OOF from recovery tests, so an Ork might spend a few activations trying to get up, but unless he gets OD'ed or auto-killed, he will eventually rise again.

(Game terms: OD = Obviously Dead, OOF = Out Of Fight)

I also made Ork specific reaction tables (which is par for the course for 5150 aliens) that essentially have them be more eager to charge and fight than they are to shoot, unless they've been placed in cover.  And even then, they might still come barging out to krak sum skullz instead of staying put.

I've been staying away from 40k Orkz, but the army profile we're developing seems really cool, so I might need to pick some up anyway.  I've always liked the Ork attitude, which is a pure "fight hard/die young/leave a disintegrated corpse" ethos, but the actual figures don't do much for me with their silly looking cartoony mouths.  Still, the Orks are archetypical fighting machines, and as such are almost the only race in 40k that isn't creepy and evil.  "What?" you say?  "Brutal killing beasts that are as much fungus as animal aren't evil?"  That's right - they're no more evil than lions.  They don't have malevolence in their hearts, just a pure desire to fight.