I went back and actually tried every app on this list Lifehacker compiled of the Top 10 iPhone Apps (top only because there aren't any others?). I found that a lot of them need some CSS work to adjust to mobile Safari's idiosyncrasies. But anyway, here's the app-by-app:
101 Cookbooks: Works great, probably because it doesn't try to do anything fancy. Unfortunately, I don't see myself needing recipes on my iPhone (or much at all).
Go Movies: Requires a zoom-in on every page to be legible.
iPhlickr: A bit more legible, but requires a zoom-in, which cuts off parts of the page.
iPhoneDigg: Requires a zoom-in to be legible. It may not be an iPhone-related issue, but I wasn't able to get past the root level.
Gas.app: Usable, but the CSS is a bit off and there's no easy way to go back to the zip code screen other than reloading the page.
Zoho: The main page suffers from the same issue as iPhoneDigg (I think it has to do with them set to auto width instead of a fixed width) and I didn't go any further because it required a login.
Google Reader: The Google Reader iPhone skin doesn't work because you can't drag the Bookmarklet necessary to do the changes to the page. Luckily the official mobile version of Google Reader works great on the iPhone, and Safari's built-in RSS reader (via reader.mac.com) is great also.
iPhoneChat: Again, there were issues with zooming. I also had trouble with the CSS not showing my buddy list in a readable manner. I didn't try IMing anyone because I just gave up after the terrible display of the buddy list.
OneTrip: This was the most refined application of the bunch, and it's one of only two that I'll keep from this list (the other is iActu). Everything worked like I expected and it remember my list even after shopping around a bit (I assume it's using cookies to store my info).
I don't see any of these as killer apps, per se, but I do hope that the developers weren't in it just to cash in on the iPhone hype and abandon these now that the iPhone is out. OTOH, perhaps showing Apple that HTML isn't a valid alternative to a proper SDK will push them to release one sooner rather than later.