We've seen them for CF cards-now, there's an SSD enclosure that will take up to six of the SDHC cards you have lying around and tie them into a single 2.5-inch SATA SSD. While it won't match speeds of dedicated SSDs (especially if the cards you're using aren't near the high end), the boys at Impress managed 111.4 MB/s read and 55.2 MB/s write times using six 8GB cards-not too shabby for a DIY solution that will save you some money (the adapter is $90 in Japan), especially if you're already swimming in big SDHC cards. [Impress]
@eben: That's partially correct. SSDs like those used as the primary HDD do have the rotating algorithm to help spread out the writes. However, the problem is the FAT table or MFT (if using NTFS)...any change to any file on the volume causes a write on these sectors, which are always in the same spots by design. A new filesystem would need to be implemented in place of FAT or NTFS would be needed to avoid this issue. In Linux, this is possible, but in Windows, there haven't been any announcements of plans to do this. Even the rumored WinFS wasn't going to deal with this issue. And you can't always relocate bad areas...some areas are restricted, such as sector 0, which is critical. Anyways, for big files, they're fine, but for small files that are often being modified, it's a real concern.
Also, the date last accessed timestamp on every file is updated whenever you even display the file in Explorer, by default. Just viewing your files will cause multiple writes in the FAT/MFT sectors.
As for the amount of time before this is an issue, I've seen quite a few USB flash drives reach their limit within a year or two, only being used with small files. This is especially true when working with many backup and temp files. What happens is the sectors that contain the FAT (or MFT) can not longer be overwritten, so it says the device is full even though you may have several GB left of unused space. You can't even rename the files. Erasing a file that has it's record in a different sector (no matter how big/small) allows the new file to be saved without an error. It's weird, and maybe you have to see it to believe it, but it's happened for a few of my customers when I was a tech.