Is anyone aware of any IF games that give the player the option of switching between the normal command interface and a CYOA style interface, or alternatively with a menu in which the options map to normal IF commands?
Not sure about the others here, but I myself haven’t seen any game that had those features that you mentioned. It’s usually either a normal command interface or a CYOA style game.
The Adventure Book system (by Jon Ingold), and the CYOA-system for TADS 2 (by Mark Musante, IIRC) allows a combination of CYOA options and a few commands with the “verb-object” syntax, I think.
I know I’m a little late to answer this, but David Whyld’s Requiem, while it didn’t actually let the player switch back and forth, had both CYOA and regular IF gameplay.
EDIT: Italics are still broken! 
On a side topic, I wrote a browser-based engine for CYOA games last year, and just haven’t gotten around to polishing and packaging and releasing it. I was going to use it for a game in the Lo-Tech comp, which I didn’t have time to complete. I think it uses a pretty slick, and works in Netscape, Firefox, MSIE, and Opera.
Seriously? Cool! If you need someone to beta test it or whatever let me know. 
I occasionally get this inexplicable urge to write CYOAs, but I’ve never been completely happy with the programs I’ve tried so far. I really prefer the simplicity of being able to do it all online, basically just entering text in a box and adding choices, but most of the sites I’ve found seem pretty much dead. And of course posting on somebody else’s website where anyone who has a mind to can screw with your story isn’t the same as being able to write a complete game yourself and distribute the file. (Of course at this point I have no idea how your engine works, but ‘browser-based’ is a start and I’d love to give it a try.)
I should find the time to finish it up. I looked it over again after making that post, and I see that there were a couple things I had left unfinished. I was planning to work on an inventory-type system and an “examine” option, which I never got around to. Right now, it would do straight by-the-book CYOA, with saving/loading to and from cookies. The pages are added to a .JS file (something akin to “Page1Text=”…") with some settings for title, options, etc. It’s not a GUI development system, but would be pretty simple even for non-programmers.
Being browser-based does make it pretty slick, though. Bad thing is, all the source is unencrypted, so people would be able to see/cheat. I figure that’s not a huge deal, but I had thought about encrypting and unencrypting the game text just to make it a little harder to peek and get spoilers from the source code for a given game.
See also my I7 extension “Simple CYOA”
I was at the Wichita River Festival this weekend (a yearly thing, here), at the Art and Book fair, where I found a box-full of classic “Choose Your Own Adventure” brand books. I found a few misellaneous CYOA-style books in addition, including four in a “Time Machine” series, a Zork CYOA, a D&D CYOA, and a “Twist of Fate” or two. Also, I found all but one in a pair of quadrilogies called “Escape from Fromme” and “Escape from Tenopia” (or something like that). They were $0.25 each, so I couldn’t pass them up. 
Ah, the good old CYOA gamebooks. I have fond memories of them from my mispent youth, though I seem to remember that as with most gamebook series the earlier ones were a lot better than the later ones.
“Escape From Fromme” and “Escape From Tenopia” sound familiar but I don’t think I ever read them. What are they about?
Looks like fantasy, but I haven’t checked them out yet. They’re still in the box I carried them home in.
My wife is a social worker in an elementary school. I think some of the kids would like these kinds of books, so I’m going to send some with her to give away. At a minimum, I’ll probably keep the Zork one.