RAID-0 may not be able to improve access times, but it can improve the transfer rate over that of a single SSD.
The huge bulk of the speedup you get with SSDs is due to their much faster access times (about 100X faster than a hard drive). But transfer rates can help too, particularly when the access times have gotten so short that they don't account for as big a percentage of the read times any more.
Applications open much faster when installed on an SSD, but they're still not all instantaneous. For example it takes my 160GB Intel X-25M about 3 seconds to open Photoshop CS4. That's much, much faster than a hard drive, but there's still some room for improvement.
Personally I'm very happy with the 3 seconds and I certainly don't feel it's worth the bother of RAID to improve it further.