Yes, and I think that this will grow in the future
I use FileBrowser to do this (SMB shares). GoodReader can also do the job via FTP.
I am reasonably sure that most if not all NAS devices support SMB shares and FTP.
Also, various manufacturers are starting to market iPad support for NAS devices. Western Digital and Buffalo are two the I have noticed recently.
Many NAS devices have browser based "file management" varying in functionality from quite restricted (e.g. WD Copy Manager, Download Manager) to more general (e.g. QNAP).