I signed up with Grand Central way back when it first launched and then transitioned to Google Voice along with everyone else. I then paid $10 for a custom Google number and love the experience on Google Android devices. I was browsing through the Google blogs and discovered that Google is now offering you the ability to use Google Voice with your existing phone number instead of having to select and use a new number. Keep in mind this is a lighter version of Google Voice as you can see in this blog post and in the chart shown below. If you have a Google Voice account, you can add Google voicemail to any mobile phone linked to your account.
Click to see larger comparison chart.
They posted the video below to show many of the features and explain the voicemail system.
The main reason I plan to add Google voicemail to my existing mobile number is for the text transcription of voicemail, but I still plan to stick with my custom Google Voice phone number because there are so many additional valuable features and using a device like an HTC Hero makes for a rather seamless experience.