I recently saw an MSOCache folder (Microsoft Office Cache), in my C: drive, and I was wondering whether I should delete it or not. The folder is pretty big, almost 1 gb. I'm an Office 2010 Beta user too. I haven't noticed this folder until today, and I haven't noticed it in my Disk Cleanup yet either, so I'll have a look there soon. Here's a picture of the folder: 
If you delete it this is what will happen:
* MS Office's repair function won't work from the hard disk cache anymore - will require the CD's to fix it.
* Microsoft Updates for Office that require the cache won't work from the hard disk cache anymore - will require the CD's to install them.
Solution, what I did recently:
1. Burn that whole folder to a CD-R or DVD (the filesize of that folder depends upon your Office version).
2. Delete that folder.
3. Search the registry in RegEdit for C:\MSOCache and change all references to point to your CD/DVD drive, example:
E:\MSOCache (will of course require the disc when something Office related needs those cache files.)