The United States has recalled its ambassador to Syria, nominally because former Lebanese Prime Minister Rafik Hariri was assassinated. The only public confession of guilt for the attack is from an extremely obscure organization that claims Hariri was assassinated because Hariri's policies inclined toward Saudi Arabia--or so I have read.
For more details:
http://www.cnn.com/2005/WORLD/meast/02/15/beirut.explosion/index.html
Diplomatically, recalling an ambassador is an extremely serious step, rarely carried out even under the most unpleasant of circumstances. For example, during the Cuban Missile Crisis neither side recalled its ambassador, to the best of my recollection.
A wide variety of accusations have been made against the Syrian government, notably that it funds terrorism, that it supports the Iraqi Freedom Fighters against the current occupation of Iraq (how tragic it is to write those words about an American Army and its mercenaries), that it has secretly received Saddam Hussein's stock of weapons of mass destruction, his gold supply, or the like.
It is plausible to suppose that an attack into Syria from Iraq would lead to a negative reaction from the newly-elected Iraqi provisional government. On the other hand, Syria is probably an easier target than Iran, especially since an attack on Syria might have support from antiSyrian armies approaching from the Southwest. However, that sort of involvement might make peace in the Middle East impossible, and might lead to another oil embargo.