Ten One Design's SoundClip promises to "pwn your sound." This US$7.95 passive sound enhancer (their description) for iPhone 3G's uses a tuned conical deflection chamber to "[direct] sound from iPhone toward you to increase the clarity and volume for movies and music...SoundClip amplifies iPhone audio by 10dB between 6kH and 20kHz, resulting in a cleaner, more accurate response."
What this means is that the SoundClip uses the same acoustic waveguide principle as the Griffin AirCurve we looked at back in October '08, only in a smaller form factor.
There are two audio clips on the site, demonstrating the SoundClip with both a music program and pure white noise. It does in fact boost the high frequencies as advertised, but the effect is more pronounced with the noise than with the music. In fact, the SoundClip FAQ admits that this won't make your ring tones louder, since they have mostly lower-frequency content.
At least they don't over-hype the device's abilities. But unless your dog listens to your iPhone a lot, this might be eight bucks better spent elsewhere.