I really don't see Spike working with Warren, he despised the Trio and they were afraid of Spike (didn't have a camera in his crypt when they were watching Buffy)
I don't think the Trio was afraid of Spike. Not when he was chipped, at least. Warren had made him a bot for him, Andrew had a crush on him, and Jonathan thought he was "cool".
To play Devil's advocate: Yes, Spike did despise the Trio -
but think about it. Spike needed money - for you know, cigarettes and blood and to help Buffy out (even though she refused his help) It's not like Spike had any other means of income. After all, before he was chipped, it was implied he'd just steal money from his victims, or make deals with other demons in exchange for cash. He had stopped that type of behavior in an attempt to change. So, he's left broke. He has gone to Warren before for "help" (to have the Buffybot made, for instance.) I honestly don't see it as being totally out of left field for Warren to have approached him, and offered him money in exchange for hiding the eggs. Spike says to Buffy in this episode, "You know what I am. You've always known. You come to me all the same." Is this admitting he was responsible for the eggs? That he is indeed the "Doctor"? Not necessarily. Really, we can't prove who the Doctor was. Could be him, Sam, Riley, or Warren. The ambiguity is delightful, isn't it?
Yet, I also see it as being highly plausible that Riley planted the eggs there, to set Spike up. I agree that it was out of character, at that point for Spike to be involved in that.
So, assuming Spike's innocent? I'm going with Sam and or Sam/Riley or Warren.
It's just....I don't get this episode. Why leave it so up in the air like this? Like they really thought all of us would believe it was Spike.
Despite, if it is Spike, they needed him to do that in order to be the catalyst for Buffy breaking it off with him. Sure, she left him because she claimed she didn't love him, but she also left him because she knew he hadn't changed. He was still a soulless vampire, and using him was killing
her. Just like they needed Spike's attempted rape and the guilt that ensued to be the catalyst that would propel Spike to go and get a soul.
Regardless of who was responsible for the planting of the demon eggs, it had the desired effect: Buffy thought it was Spike, broke it off with him, and Riley got what he wanted - for Buffy to leave Spike.
Stupid Riley.
So, I guess what I'm saying is as much as I'd like it not have been Spike - I can see the arguments against him being the Doctor and the arguments for. I guess we'll never know. I wonder if Douglas Petrie had anything to say regarding this.
EDIT: I would also highly suggest everyone whom hasn't had a chance yet to read that article that was linked to earlier. It's really interesting. The author brings up some great points.