If crocodillians and birds make up the "archosaur" classification, does that mean that crocodillians are more closely related to birds than they are to other reptiles? If so, why are they still classified with reptiles?

You are exactly right, Megan. Crocodilians are indeed more closely related to birds (and dinosaurs) than they are to other modern reptiles. Crocodilians and reptiles used to be classified together in the old Linnaean system probably because of the superficial characteristics that they share (quadrupedal locomotion, scaly skin, etc.)

Well it's more that reptiles in the traditional, or rather, common sense (crocs, snakes, lizards) is not the same as the scientific one. Reptiles (or Reptilia) are still a valid taxonomic group, it just also includes mammals and birds.

Also, don't be fooled by the term "more closely related to" (or for that matter "closest relative" or anyting simialr). This is a phrase used to describe evolutionary relationships rather than actual physical simialrities. For example we say a trout is more closely related to a human than a shark. That's because if you just deal with these three things, the trout branched off from the main vertebrate tree after the shark and thus is 'closer' to humans. Of course it would seem that sharks and trout have more in common (like fins and gills etc. etc. etc.) hence the apparent trap in this phrase. In many ways crocs are *literally* closer to lizards (the scales, low metabolism, etc.) but evolutionary yes, they are closer to birds.

Since when did Reptilia contain Mammalia?  I understood it to mean (if interpreted as a clade at all) the same thing as Sauropsida, i.e. everything on reptile side of the reptile-mammal split.

... but of course redefining a term like Reptilia as a clade is silly anyway.  Much better just to use the informal term "reptile", since everyone knows what's meant by it, and when you need a formal clade name, to use something like Sauropsida that has no informal meaning to muddy the water.

Whoops, yes sorry Mike, I was thinking of Sauroposida and not Reptilia.

Though it is a valid and used clade: http://tolweb.org/articles/?article_id=462