The Camera RAW update, ACR v5.5, in the earlier reply won't work with PS/CS3. Don't even try to use it in CS3.
If the OP needs a newer Camera RAW than the last of the ACR v4 offerings there are two choices:
1. Download the free Adobe DNG Converter and use it to convert the camera specific RAW files into Adobes standardized RAW format. All versions of PS since CS can read the resulting DNG files (PS/CS does require at least ACR v2.2). Links:
Windows:
Adobe - Adobe Camera Raw and DNG Converter : For Windows : Adobe DNG Converter 5.5 update
Mac:
Adobe - Adobe Camera Raw and DNG Converter : For Mac : Adobe DNG Converter 5.5 update
2: Upgrade to PS/CS4 and then update the ACR v5.0 that ships with CS4 to v5.5. PowerMac users should consider avoiding ACR v5.5 and only updated to v5.4 until v5.6.
If you aren't regularily having to deal with RAW files newer than your CS3 can handle, the free DNG Converter is probably the better route. Its free. It is an extra step, but can do batch processing of whole folders so its not too much of a chore.