Like @bmargulies said, just use svn.exe
:
cd "c:\svn\MyProject"
svn commit -m "Files Modified 2010-02-12"
You cannot hide the TortoiseProc.exe dialog on commit:
Important
Remember that TortoiseSVN is a GUI client, and this automation guide shows you how to make the TortoiseSVN dialogs appear to collect user input. If you want to write a script which requires no input, you should use the official Subversion command line client instead.
(from http://tortoisesvn.net/docs/release/TortoiseSVN_en/tsvn-automation.html)
If you're relying on TortoiseProc's /closeonend:3 behavior to alert you when there's an error, conflict, or merge, you'll need to switch on SVN's exit codes.
I couldn't find a list of return code values beyond "0 is success, not 0 is error" in a quick Google search, though.