And here is the deleted scene I promised. It should clear up what appears to be a minor plot hole in the main story.
XXI. I Wanna Mean It from the Back of My Broken Hand
How the Rose and the Note Got onto Buffy's Bed
Why Darla Has Little Patience for Penn and Spike
"Bollocks," said Spike. "Didn't think of this."
"We could just tell Darla we did it and find some other way to capture the Slayer," said Penn. The two of them were perched on the roof outside the Slayer's open bedroom window, facing the dilemma of planting the rose and the note Darla had forged on her bed—without having been invited in.
"No," said Spike, "this way is better. Poetic, you know?"
Penn scowled and rolled his eyes. "Then what do you suggest?" They could hear the sounds of voices and the chink of knives and forks on plates coming from somewhere inside the house, but the Slayer wouldn't be eating dinner forever. They had to think of something fast.
"Fishing pole?" Spike offered.
"Yeah, that's great!" said Penn, clapping Spike heartily on the back, making him stagger where he crouched. "Have fun getting to a sporting goods store, burglarizing it without anyone noticing, and making it back in the next ten minutes. I'll wait here."
"Let's hear your suggestion, then," said Spike crossly, banging Penn's head against the invisible barrier keeping them outside. Penn shoved him in retaliation, almost causing him to topple off the roof.
"Give them to me, I'll just toss them in."
"No, you great sodding prat," said Spike, holding them out of reach, "what if you miss the bed? Here, let's just find a long branch with some twigs at the end."
With this solution, the task was accomplished fairly easily, though Penn accidentally bumped Spike when he was almost done, nearly causing him to drop the branch inside the Slayer's room. Spike whacked Penn over the head with it when it was safely outside again. The two vampires continued inflicting minor injuries on each other all the way to the apartment where they were to lie in wait, only ceasing when they heard the Slayer approaching.
So, yes, the note was forged. Darla had been spying on Buffy and Angel enough to know about their French endearments for each other. It's really quite a pity that Spike and Penn never got a chance to interact in canon. I'm pretty sure it would have gone a lot like this. It's also a pity that they never did the "banging a vamp's head against the invisible barrier" gag, because that would have been pretty hilarious to see.