I recently got an SSD drive for my primary boot drive. It was a moderately fast one, with consistent 270mb/s read and write speeds. I've used lightroom with the catalog both on the SSD and on a normal HDD, and I did not see a whole lot of performance improvement for my catalog, which is about 12,000 photos or so.

As I started investigating how to improve the performance of Lightroom, I realized that the most significant factor that affected performance was the Camera RAW Cache size (Preferences -> File Handling -> Camera Raw Cache Settings -> Maximum Size). It defaults to 1Gb, however when working with a large number of RAW photos, a 1Gb cache is FAR, FAR too small. Increasing it to at least 20Gb offered good improvement, and increasing it to 50Gb offered considerable improvement. The improvement was a bit better on the SSD, however given the value of SSD space, I again moved the cache off onto a normal HDD. The performance gains are still evident, and I'm quite happy using LR with all of its data on a normal hard drive.

It should be noted that SSD's come in a very BROAD range of speeds and capabilities. Some are fairly slow relative to an HDD (which can usually offer around 60mb/s burst rates), where as some are insanely fast. SSD burst rates can range from as low as around 100mb/s or even lower for really cheap drives, to as high as 700mb/s or even more for highly specialized designs...such as those that fit into a PCI-E slot. Some SSD's include built-in high-speed secondary caches, some have better data distribution algorithms that optimize read or write speeds, etc. All these factors can affect whether an SSD might show a significant improvement for Lightroom performance or not. Lightroom generally stores the working image in memory, so if you are worried about single-image development speed, an SSD is not going to help much. The performance gains that can be realized from faster storage drive speeds happen when moving between files, performing multi-file operations, etc. The bottom line is...the more money you are willing to spend for better SSD technology and faster burst rate, the more likely you are to see an improvement.