The Jamestown Foundation stumbled across something interesting here: a Syrian website puts out a call for mujahideen to come to Iraq through Syria. It starts off by noting, "the situation of the mujahideen in Iraq is entirely stable, and that they are not suffering at all from any shortfall in mujahideen," and says they don't need any more inexperienced fighters. Apparently they've got all the rubes they need for suicide bombings. But it also warns fighters to be wary of the Bashar regime in Syria.
Obviously who knows how reliable this is, but it's one small piece of evidence that: a) the foreign fighters in Iraq feel confident about their strength (though they could well be bluffing), and b) the Syrian regime doesn't appear to be aiding and abetting the foreign component of the insurgency. (I believe there's still some question as to whether Syria is harboring some of the ex-Baathist leaders running the native Iraq bit.)