Enter Cadence (US$4.99). It scans your library and lets you create and select playlists that match particular BPMs. The app itself is simple: there is a slider for choosing a particular BPM, and a rudimentary play/pause/skip button array at the bottom of the screen. Helpful features behind the curtain include a tolerance setting, in which you can have the app pull in songs that are slightly above or below a certain number of BPMs for greater variety. In fact, the lyrics at the beginning of this post - Check Yes Juliet by We The Kings - was one such song serendipitously served up mid-run in the 165 BPM (+/- 2 BPM) category. And yes, my pace picked up considerably.
Setup, however, isn't particularly easy to figure out. Rather than a button called "Scan Library," you need to go to "choose BPM source." Once I was there, it took the iPhone three times to effectively do that, stalling twice. If you've downloaded the app directly to your iPhone, you can only import songs that are part of a known BPM database; categorizing your entire library requires their desktop app, Cadence Desktop. Unfortunately, and curiously, Cadence Desktop refused to give Check Yes Juliet a BPM setting at all.
A few more drawbacks: the app doesn't work with the pause/play/skip button on my earphones. Since my iPhone lives in an armband while I run, I rely on the earphone clicker to navigate my music. Second, while the native iPod app will work with other apps, leaving your iPhone free to, say, run a GPS program such as RunKeeper in the foreground, Cadence takes the place of that foreground app. Perhaps with the advent of multitasking, this problem will be short-lived. Until then, I'm using my trusty Garmin Forerunner.
It also doesn't appear to be a particularly stable app; sometimes when you change BPMs, the song titles disappear until you reboot the app. Sometime the app itself won't load at all. Other times it crashes, stalls, or takes a while to load up the next song. And, in iTunes, Cadence Desktop creates almost 100 smart playlists, creating a lot of visual clutter. That being said, if you are having trouble using the Cadence app itself, using its desktop app to categorize your smart playlists would ostensibly then free you to choose these playlists directly in the iPod app, avoiding multitasking issues as well as app instability. You lose the smoothing features and the slider, but if your app won't run at all, it's one way to get out the door and on your run. That, after all, is the whole point.
Update: according to the developer, the issue with Cadence Desktop not recognizing the song in my library was that it was a DRM-protected song, and a problem common to all programs that seek to analyze DRM-protected music. BPM information can be added manually, though this in general is not a great solution for large libraries amassed prior to DRM-free iTunes downloads.