1) I'd say it depends. It's definitely feasible, but it depends on what is in place to see unusual conditions...I mean, are you saying unauthorized access as in physical, or they broke into a server? Physical access could mean adding another machine to a switch port and mirroring data or stealing backup tapes, and if it's remote then most ISP's should notice a spike in outgoing traffic choking their connections as data is being sniffed and mirrored out (or spikes as data is transferred later). Plus you have to redirect traffic through a system that is not a router ordinarily in order to see the traffic, since without setting up a switch to mirror traffic you have to spoof ARP requests to redirect gateway traffic. But yes, it's possible they could sniff data.
2) VLANs, updates on systems, auditing for unusual behaviors, etc.
Are you asking as a user afraid of what the ISP is doing to watch your traffic? You're never fully sure your traffic isn't being watched. I mean, think about it...you are doing banking traffic. Your traffic is going from your computer to your provider to your providers upstream provider (and you don't know how many hops that is) to the ISP of your bank to your bank's servers. Your connection stands to be hijacked at your network, your ISP, your bank's ISP, and your bank's employees, and any hop on the backbone in between. And how do you guard it? Encryption, at most. That's assuming someone isn't injecting a fake cert to hijack your SSL connection, and that no one has installed a keylogger or screen capture program on your system or a bank employee's system.
Can someone do it? Sure. Odds are you'll have a thief just as soon mug you at the ATM than go through the trouble, but yes, it's possible. It's just not a threat so common that it's worth losing sleep over.