Hi, Alex!
There is one being developed: Gvoice (http://mytreo.net/forum/index.php/topic,68662.0.html)
I just installed the latest version of it and and it works. I encourage people to try it and make a donation to the developer.
Gvoice authenticates to your Google Voice account one time. After that, when you dial your contact's phone number, it apparently sends the information to your GV account which dials back to your cell phone, then connects to your contact.
The developer has recently added some nice features, like cut and pasting phone numbers from other sources. He has also included an SMS feature, but I haven't tried that.
Gvoice doesn't really suit my needs, so I don't use it. I've only installed it out of curiosity. I'm keeping an open mind about it. I would really like to see a GV app for PalmOS that works more like Takephone's credit card dialer or the Palm's speed dial.
To set up speed dial for GV, simply create a Favorite for your contact. Put your GV number in, then in the extra digits page, put '<pin if needed>,2,<your contact number>#'. You can play around with commas for extra pauses (or the letter 'p'). You can find some discussion about this at Brighthand (http://forum.brighthand.com/showthread.php?t=269001).
I haven't bothered with the GVoice app's SMS functionality because it works so flawlessly for me through the 755p's Messaging app. I send a text message to a friend asking them to respond using my GV number. Their message comes to me with a GV '406' number which I program in for that contact. From then on, all I need to do is send them text messages to that '406' number and it arrives to them as if it were from my GV number.
Using the above methods (plus a few more I'll mention later), my Google Voice number has become my exclusive phone number. All incoming and outgoing calls and text messages on my cell phone are through my Google Voice number.
I use Sprint and have their Plus 3 feature turned on. My GV number is one of my "favorites." I also have (free) conditional call forwarding set up on my phone so all incoming calls that I'm unable to answer are routed to my GV number instead of my cell's voicemail. I have talked on my cell phone over 500 minutes in just over 2 weeks, yet my minutes used on my Sprint account shows as zero because everything goes through my GV number.
Okay, I'm bragging. Sorry.
The majority of calls I make are through the Favorites I've set up, and the majority of text messages I send use a '406' number. Once in a while, I need something else. Options include calling my GV number and dialing directly through that. My preferred method is to simply go to my GV account's contact page through Blazer, find the contact I need, and dial (or text) through the web page. Very easy.
Still, I'd like a cleaner, more consistent dialing method, preferably one that uses Palm's contacts. TakePhone's credit card dialer was very promising, but at the time I was experiencing too many misdials, similar to what I experience with my Favorites method. If it had provided a significant improvement over my own method, I would've paid for it. I'm guessing the misdials have something to do with DTMF tones getting ignored or somehow out of order in low reception areas. That's the best guess I have, but I haven't been able to consistently repeat the problems I have, so it's difficult to test. Since the problem seems to happen less often in recent weeks and the majority of my calls aren't critical in nature, I just ignore it. I often wonder if I'll have this issue with my next phone (hopefully the HTC Touch Pro 2). :)
Note that Gvoice gives some access to Palm's contacts for dialing, but I think it needs a little work. If that functionality were cleaner, I might be more interested. Come to think of it, I think the developer may have added a speed dial feature. In fact, I do remember seeing a new calling history available, which I think you can use for dialing. When I have more time, I'll check out that out a bit more because that would get me a bit closer to being a Gvoice convert.
Take care! :)