My experience is with Snow Leopard and Windows XP machines. Given that, I say yes, you can use a Time Capsule (TC) for backups and simultaneously as a simple NAS to share files between OS X and Windows XP (and I assume, Vista and 7).
I've successfully shared plain text files and OpenOffice documents between Mac and Windows this way.
But there are a few things to take into consideration. One is the issue of the backup disk filling up with TM backups. This is not a problem for TM, but you won't be able to place any more files onto the TC disk until you delete some old backups manually and this isn't desirable.
The best solution is to use the TC internal disk for TM backups only. For file sharing/transferring, plug an external USB drive or a thumb drive into the TC USB port.
There are other workarounds. See
Can I use my Time Capsule for backups AND other data?
Also, note that any file you put onto a TC disk will
not get backed up. And, I've read that TC is pretty slow compared to other NAS devices (although that wasn't a problem for me).
Regards,
Brian33