Warp Scream and Grenades

That's a good question. I would say yes, but that's just my take. I don't have the codex infront of me.

This is a bit of an odd question. First of all plasma grenades ignore the I10 cover bonus. So you would fight at normal initiative when faced with these. From the letter of the rules (in the BBB, not the codex here) it would seem to suggest that you can- there are all manner of abilities and rules that change this that aren't grenades, so why would warp scream be any different?

http://www.librarium-online.com/forums/showthread.php?t=47170

Try looking here, might shed a litle more light on the subject. Actually it sheds wuite a lot of light but still, it's down to personal enterpretation.

Hmm... this was a really good screw-up by GW. I guess I would go with Noise Marines striking first, and I am sure my EC-friend won't argue

I personaly play with warp scream altering I and then cover bringing it up to 10. It seems silly that a unit that would strike first in the open, say genestealers, is penalised for being in cover. If the warp scream alters I after the cover gives the defender 10, then Noise marines always strike first in cover weather defending or attacking which seems like a loop hole. When this has come up against opponents I have rolled off for it, as there is nothing to prove one way or the other, but I don't like playing games against people who look for every last loophole abuse, and won't cut them any slack.

So you're saying that in this instance, against plasma grenades that warp screm would have an affect? I agree with what you're saying though about penalising things that would react faster in combat anyway.

Luckily for you you are one of the people with a fairly reasonable-sounding game=ing group. There's people I know who will argue the toss about the most ambiguous rules and you just lose the will to live.