I welcome another streaming service. Why? Because it increases the alternative options for cable customers. Consider:
Netflix ($9) + Hulu Plus ($8) + Amazon Prime ($8.25) + Sony ("competitive") = ~$35
Comcast's Digital Preferred (where I live): $86 (not counting taxes) + $10 HD fee + $8 DVR fee = $104
There's some overlap with the streaming services, but each one offers something a little different, and customers can get a pretty broad selection. Combine with ad-hoc Redbox rentals for hardware viewing of recent releases and the upcoming HBO Go offerings, digital cable television as a value proposition becomes less and less appealing for many customers.
The single biggest draw for cable IMHO is live sports - and ESPN currently offers a similar deal to what HBO is moving away from. I think as internet streaming reaches critical mass, content providers like ESPN may decide that their exclusive contracts with cable companies are a liability. Another important issue is advertising dollars - cable-company provided DVRs allow viewers to skip commercials, whereas live streaming doesn't currently allow that, so commercials on streaming video may start to look more lucrative than on TV.