If anything, there seems to be a resurgence in writers publishing under pseudonyms for different genres and even sub-genres. Mark Barrowcliffe writes (or wrote) mainstream fiction under his own name, and writes fantasy as M D Lachlan - and his new series, which is still fantasy but different in tone to his Wolfsangel series, is coming out under yet another pseudonym (Mark Alder). I know other writers (some of whom I can't name, since I was told in confidence) in similar situations, so this isn't a one-off.
The publishers have to balance the benefits of an existing brand against the likelihood (or lack) of a crossover in readership. They want to prevent the casual reader from having wrong expectations when picking up a book, probably even more than they want to exploit a known brand, especially if that brand isn't mega-famous. If you're not JKR or Stephen King, most likely you'll be expected to use pseudonyms.
I think you'd have trouble selling one SF book and then a fantasy, for example, but get a trilogy under your belt and genre-switching is more of an option, particularly if your first books sold only modest amounts. A poor track record of sales can be a serious disadvantage in today's computerised ordering systems, and a new pseudonym can get you round that!
As for agents...it depends. Some agents are very genre-specific (John Jarrold, for example), others are a bit more eclectic - some authors have multiple agents for different genres, for that reason.
