Mr. G: From what I gather, both from editors and writers I've talked to, when it comes to houses like Penguin, etc., while errors do creep in more frequently than they used, they still have a rather higher standard when it comes to the literary canon. They are somewhat less rigorous with modern "classics". And mainstream literature -- that is, modern non-genre literature -- suffers from a lot of the same things any genre does: the lack of proofreaders, haste and carelessness in editing, etc.
However, in my own experience... yes, genre literature does seem to have more than its share of such problems, especially when it comes to sff. Nor is this only the small presses -- I see a fair amount of it in the major houses, as well. Even some of the small presses which are normally quite careful about such have some odd things; I recall, with Hippocampus' complete set of Smith's verse, places where the notes or introductory matter would direct you to "pp. 000-00"... the sort of slip-up for which, during my years as a typesetter/proofreader, you'd have been called on the carpet and left the office with your ears ringing!
However, this is nothing new. I love the old Zebra REH line, but mein Gott! the horrible typos, problems with paste-up (entire lines or even partial paragraphs being repeated at the bottom or top of pages, bits of correction which had slipped and been shot at an angle, where they blocked parts of several words, etc.) and the like, were simply atrocious! I don't know if I've ever seen the like from any professional publisher before or since. But I have seen close to it at times; so this trend dates back to at least the mid-1970s.
(On the other hand, there are small presses which are almost obsessive about taking care of such things, and these generally also have higher standards in most other regards as well.)