Giggles and poops in wotlk

For poops and giggles, if I stack +Spell Power in my rogue, will my poison damage go up?
I actually don't think it will. If you asked that question a few months ago I would have said yes, but as of Wrath, poisons scale with Attack Power, not Spell Power. I think the top rank of Instant Poison gets 10% of your AP, but not benefit from spell power. All of the poisons have their own coefficients, I just picked the least mathy as my example. All scale with AP.
Has there been any word from Blizzard about increasing the daily quest cap? I'm a casual player and so only at 75 so far but I've already got a list of about 20 daily quests that I could run through to help push my rep and xp (leaving more quests at 80 to try and pay for all the really expensive shinies I can't justify right now) and I'm sure there will be plenty more coming in all the zones I haven't touched yet.
No, we haven't heard anything like that. That doesn't mean they won't do it, but we haven't heard anything about it. In my opinion, I think you would benefit from not worrying about most dailies until 80, and just do the ones that give you reputation for now. Regular quests give more gold when you're level capped but so do dailies, so what order you do them in isn't a huge impact in the long run. I'd think you would want to avoid having your leveling become horribly grindy though, and there are loads of quests. Even when you hit 80 you'll probably have a few whole zones left to run. Try not to focus on dailies too much so you don't burn yourself out before you get to see Icecrown, eh?
Why did Blizzard, again, the main city to be shared for both factions, even when they know we will have double population, and double lag?
Before I recommend a system upgrade, do you have Shadows turned on? If so, how high? If you have the Shadows slider all the way up, take it down a notch and boggle as you gain 20+ FPS instantly. Very few systems can actually handle the slider being all the way up, because absolutely everything is given a dynamic shadow. Characters, NPCs, buildings, doodads, all of it. It's pretty, but that's intended for very high end machines. The next step down is still very pretty, but gives you a huge boost to your FPS.
If you don't have shadows turned on, you probably should look into upgrading parts of your computer. Without knowing what you're using, I can't make any recommendations beyond making sure you have enough RAM and seeing that your video card isn't excessively outdated. My computer has a brand new CPU and 2 gigs of RAM, but the video card is four years old. Dalaran still goes at a good 30-35 FPS for me, which is lower than when I'm soloing but higher than when I'm in a raid. A new video card would make it run like a dream. I'd like more RAM as well, but it's lower on my upgrade priorities. Dalaran's not that bad on lag at all, especially when you consider a majority of the Horde and the Alliance are split off into two totally opposite ends of the city. I actually think it's better than Shattrath ever was.
I have a Shadow Priest I leveled to 34, then had her boosted to level 60 through Recruit-a-Friend level grants. She's now stuck in level 25-30-ish questing greens and SFK blues. I'm wondering what would be the best way to gear her. Would she be able to survive questing in Outland in her very low-level gear, or am I going to have to round up some gold for her?
If you're very careful while doing the first handful of quests, you'll be fine. Take it slow, play it safe, and after your first batch of quests you'll have a few pieces of new gear, which means more Stamina, which means you'll be okay. If you don't want to putter around that long, just hit the auction house. You don't need to spend a lot of money, just buy a couple of pieces of gear with extra stamina. That's pretty much the only thing that will cause an issue that early on: Your HP. If you only have 3,000 hitpoints or something like that, you'll be insanely squishy. As long as you have the hitpoints to survive getting an add or two, you're perfectly safe. Otherwise, time to hit the auction house.