Appleinsider claimed they'd be accessible from a browser but I haven't seen that reported anywhere else.
The way it seems to work is that you're asked if you want to store them on your device and cloud or keep thumbnails on your device and store hi-res versions on iCloud. Since we haven't seen a Photos For Mac beta I'm not sure how it'll work on there.
Hmmm that's too bad we haven't seen a BETA of Photos yet because that all I really care about. Getting the photos / videos from the phone to the Mac.
On second thought, if iCloud storage becomes competitive with Drop Box, I may not even need them to be stored locally, however, I like having a library (locally) because I have Time Machine and CrashPlan backing it up.
I guess it's just back to the waiting game because everything is speculation right now.