Will mounting my disk as "ro" make it completely read-only?

If you mount a filesystem read-only, there is no way to modify it through filesystem operations. Not even if you're root. If you want to perform any modification (write to a file, remove a file, change permissions, update the access time, etc.), you have to remount the file system read-write (mount -o remount,rw /dev/foo).

You can still access the underlying device and write to it. (It's a very bad idea, of course.) Disks and partitions are only accessible by root unless you've explicitly changed this (usually a bad idea too).

One thing that doesn't come up very often is that the action of mounting itself might write to the disk. This can happen even with read-only mounts for journaled filesystems: if the filesystem was not cleanly unmounted, the action of mounting the filesystem may replay the journal and perform the queued actions (it does for ext3; I don't know about ntfs). This means you can't easily inspect the disk of a suspended machine (e.g. the disk image of a paused virtual machine viewed on the host, or a hibernating system from a rescue CD).