1) A slide-ruler. Something letting me pinch-zoom in/out on any point of the ruler, and I can slide the middle/top/bottom rules by swiping my finger on that rule. Yes, I know iPhone has a built-in calculator, I want a slide-ruler and am willing to pay. Give it all 7 (or 10, or however many) rules, with the ability to slide them to damn near infinity (or at least 10 or so repetitions), and I figure this would be a good solid $5 app.
2) A *complete* dictionary, with a reasonable interface. I'm talking unabridged, with etymologies and historical quotes, synonyms/antonyms, pronunciation guides, etc. Basically what you'd get from OED online and a good thesaurus wrapped up in one, but offline in an app. Give it a good interface, with the ability to jump to definitions for any word used in the current definition, and don't waste screen real estate with meta cruft. If you want audio pronunciations that's fine, but I'll likely not use them. Pictures are nice but not needed. I figure this would be worth at least $30 or so, possibly $50 if it is done well enough.
3) An encyclopedia. Britannica on my iPhone? Yes please! Right up there with the dictionary, another app that if done well could fetch $50. Combine them into one big app where everything is cross-referenced and words used in encyclopedia entries can be defined and items used in definitions or sample sentences can be found in the encyclopedia, and you've got a killer reference app. Sure, it'd probably take up gigs of space, but that's okay since I've got gigs free (you'd probably want to keep it under 2.5GB, though). You could probably make a "compressed" version that leaves out all the images and does compression on the text to get the size down to only about 1.5GB. It'd run a little slower (decompressing on the fly and all), but would take less than half the space.
These are three apps I keep coming back to wanting on the iPhone. Slide ruler, just because I like them. But the reference apps are two things I use the phone for a lot, but there are quite a few times when the cell signal is just sucky enough to make it useless. Wikipedia only works when you're online, and it's sometimes far from authoritative anyway. You can't use google searches for defining words if you can't search google, and they don't give you the full joy of the unabridged OED even when they do work. I do this often on my phone and would be willing to pay good money for this killer app (or apps, if they can't be combined), and I can't be alone in this.