The only downfall is that only 10 seconds of data is stored but as the developer mentions, this will be increased in the future releases. Moreover, you can submit the data to the developer website for future retrieval.
All in all, iSeismometer makes your iPhone even cooler toy and if you live in a seismically active part of the world, this iSeismometer is a must to have application. Moreover, iSeismometer is FREEWARE!
You can download it from this link: http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=304190739&mt=8