This is built into Acrobat depending on what actual meta data you want to modify. I'm using Acrobat 8 and it's pretty simple:
1. On the upper menu bar, go to Advanced -> Document Processing -> Batch Processing.
2. This opens the Batch Processing dialog box. Create a New Sequence. Give it a name.
3. Now you are looking at the sequence editor. Choose Select Commands.
4. From the list of commands, choose Description and Add to move it to the left side.
5. Double click and change the author, title, keywords and all that.
Now this only works for simple cases. If you want to handle more complex cases where the keywords and such will be taken from the document itself, you'll need to do some scripting. I don't know how to do that. I've done a bit of InDesign scripting and, like MS Office applications, the Adobe scripting options are powerful, but there's real programming involved and often a relatively esoteric API.
Whether or not someone has created an app that will already do this dynamic stuff, I don't know, but since extracting data from the document itself will be highly variable and depend on how the document is constructed, it seems like it would be a challenge.