The Bush administration undoubtedly overreaches when it contends that the President's power as Commander in Chief precludes Congress---which has more enumerated powers relative to war than the President does---from "micro-managing" the war through troop withdrawal deadlines and the like. Yet surely the administration has a point (albeit a different one) in objecting to Speaker Pelosi's acting as messenger to Syria on Israel's behalf at a time when the State Department opposes direct talks with Syria. (Whether that opposition makes sense is a different matter.) The Constitution's Article II gives the President the power to "receive Ambassadors and other public Ministers," while providing Congress with no parallel authority for conducting diplomacy---other than the power to confirm U.S. diplomats.
The conventional wisdom holds that the government should speak with one voice on matters of foreign relations, and having Congress conduct its own diplomatic missions certainly contradicts that wisdom. To be sure, it's not obvious that the conventional wisdom is right. For example, recently the administration has been using congressional threats of funding cuts to Iraq and Pakistan as a bad cop to its own good cop, and even if the tactic fails in these particular cases, it can be generally useful. That strikes me as true even if the good cop and bad cop are not simply playing a game but are in fact pursuing different policies. The resulting ambiguity can turn out to be productive.
But even if the conventional wisdom is therefore sometimes wrong as a matter of policy, it looks right as a matter of constitutional interpretation. The Constitution is best read to forbid congressional freelancing (to be distinguished from such things as congressional factfinding missions to foreign countries for the purpose of oversight of appropritations and related matters). Speaker Pelosi may undermine her public position on Iraq---where the Constitution clearly contemplates a substantial role for Congress---by asserting authority in an area where the Constitution truly supports Presidential prerogrative.