Again, and I say this as a Google Voice user, I don't think GV is a huge threat to wireless carriers. While, yes, GV can cut down on your incoming minutes since we all LOVE our cell phones we'll still have them as a primary way for GV to reach us. I'm happy to listen into someone leaving a message on my Google Voicemail and that, typically, uses my wireless minutes. It's probably because I don't have a direct line to reach me at my place of work or a landline at home. Now, if I divert my voicemail and read the transcription (which can be pretty spotty) that will cut down on my incoming minutes.
I think a lot of people just want the flexibility of GV and how that plays into an already interconnected lifestyle. That lifestyle will only benefit the wireless carriers (till some other wireless data network comes into play and lets me VOIP directly through GV through my handheld).
I look forward to a few cosmetic updates to the GV mobile webpage and I'll live without a free push of my incoming SMS to GV.