[spirochete]

The formation of a tosylate ester using tosyl chloride leaves Chloride ion as part of the product.  If the tosylate ion is such a great leaving group, why doesn't chloride then act as nucleophile in an SN2 reaction?  Is this second reaction simply too slow, so there's time to isolate the ester and add a different nucleophile?

[movies]

Yes, since chloride ion is not a very strong nucleophile.  Tosylates can be displaced by bromide or iodide rather easily, but it requires some heating usually, while the tosylates are usually generated at about room temperature.

[spirochete]

Before I read your post I went and asked this question to my organic professor.  He said he heard somewhere that the pyridine base precipitates out of solution with the chloride after taking up a proton.  He didn't sound super confident about it though.  Your answer sounds reasonable too.

[agrobert]

Yes it is common to run S_N2 reactions with a tertiary amine or non-nucleophilic base to soak up HCl as R₃NH⁺Cl^-.