I did not realize lack of a parliamentary majority dictated his cabinet. - Edit 1
Before modification by Joel at 09/03/2012 12:29:35 AM
Specifically, what you said about the French presidency being a far more powerful position if his party also has a parliamentary majority. The US president is in a similar, if not identical, position (hence Obama screwed the pooch in not taking advantage of the Democratic House and Senate majorities while he still had them.)
Indeed. Still, there's a big difference in the sense that the US president can always appoint his own cabinet, regardless of his party's position in Congress. A French president can indeed appoint a PM pretty much at will if his party controls the parliament, and has at least a strong voice in who will get which ministry, but if his party doesn't control the parliament, whoever does (or can create a coalition that does) can become PM and the president's say in the division of the ministries becomes essentially non-existent. At that point the president is left with little concrete power, and reduced to much the same position as the kings and queens of constitutional monarchies, or the president of e.g. Germany. He has a bit more power than those - I believe he still has the nuclear access codes, and might even have a veto of some sort - but not much more.
Do NOT provoke the guy with the Football.
That situation does make the presidency rather anemic absent a parliamentary majority though; unless he does, in fact, retain veto power it is hard to see how it is anything more than a purely ceremonial position. I suppose it must be possible for a president with a majority to annoy his own party so much that they decide to stop following his lead, vote in their own PM and bring him into much the same position, but I don't believe that has happened yet...
Without knowing more, my guess would be no one has enjoyed single party rule long enough for discontent with the party leader to reach that level and/or MPs lack latitude to secure re-election by opposing their party when constituents do. Honestly, if not for two party rule I am certain the party would have expelled many "Democratic" Congressmen long ago; as it is, Dems live in fear of that because nearly everyone who leaves the party does so as a preface to joining the GOP.
It is more a matter of the PM being both a member of the legislative majority AND head of state. In some ways that gives Britains PM greater (if more precarious) power than the US president. The PMs situation seems more like Obamas before 2010 or Clintons before 1994; he may need the Whip to keep a few unruly party members in line, but seldom faces an openly hostile majority (if only because such majorities tend to relieve him of that burden, by bringing down his government. 
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)Very true. Minority governments can happen, but they tend not to last very long as it only takes one lost vote of non-confidence to put an end to things. Yet another difference with the US, that... a PM can be dismissed by a simple vote of non-confidence in the House, at any time, if a majority can be found for it.
True, but both our approach and theirs are something of a mixed blessing. Impeachment DOES provide recourse for removing a president prior to the next election (one can only imagine how American government would have fared had Nixon been able to serve the last three years of his term after Watergate was discovered.) Unfortunately, since party tickets were only a constitutional amendment after the impeachment process was codified, its effectiveness has been diminished. Even when the president faced impeachment for covering up an attempt to rig the election AND the VP resigned amid bribery charges, Nixon just adroitly appointed Ford to succeed Agnew, resigned and received a full pardon for "all crimes he may have committed" (a blank check that went far beyond Watergate; that language from a member of the Warren Commission is very intriguing.) Even in that extreme case very little changed, and in any successful impeachment the presidents deputy would simply assume the presidency in his place.
On the other hand, perhaps that is well, since the head of state can only be removed for "high crimes and misdemeanors" rather than partisan or policy reasons. Removing Chamberlain on the eve of WWII was a necessity; removing Churchill in the middle of it would have been a disaster—but perfectly feasible under federal law.