At the risk of contradicting my current post above, Zarqawi could be *anywhere*.
I expect him to be in the lawless part of Southern Lebanon.
But Al Qaeda has probed several countries with attacks (e.g. subway bombings in the UK, hotel bombings in Jordan, attempted attacks in Saudi Arabia, resort bombing in Egypt, border attacks in Pakistan, suicide attacks in Afghanistan, piracy off of Sudan, etc.).
If Al Qaeda perceives an exploitable weakness from any such probing attack, then I'd expect Zarqawi to rush to exploit it (because Al Qaeda needs a "success" to overshadow their recent string of un-ending failures).
Certainly Al Qaeda's "great offensive" in Afghanistan turned out to be a flop for them this year, and if Israel withdraws and disengages (and builds the wall) in the West Bank, then Al Qaeda will have lost one of its prime Palestinian recruiting grounds.
I think that it's becoming undeniable that Al Qaeda has failed in Iraq, too...though I expect the dust to take a bit longer to settle there.
The Jordan hotel bombing backfire, the failure of Al Qaeda in Iraq and Afghanistan, and the need for "popular" victories leads me to think that Zarqawi will seek to coordinate Hezbollah, Al Qaeda, and Hamas attacks in Israel...and the best place to do all three is from Southern Lebanon.
I'd say it's a bit early to make that call. Attacks always pick up as spring advances, and we're only in early April. But I do hope you're correct.
Putting on my kingpin terrorist head scarf, I'd be hesitant to operate from southern Lebanon. One, what you say makes sense from a geographic perspective. If you and I know that, then so do all the smart people looking for Zarqawi. And Zarqawi knows that. Plus, southern Lebanon severely restricts his movements. Water to the west, Israel to the south, Syria to the east and north (which has some potential for safe haven, but you've already mentioned the problems for AQ there). Jordan is fairly accessible via northern Israel, but I just don't see him penning himself in like that.
I think he is in Iraq....still.