Earlier today, Google made a mobile web version of Google Voice available for the iPhone. To understand the significance of this move, here's a bit of back story.
Apple pulled all Google Voice related apps from the App Store back in July, which led to an FCC inquiry, which led Apple to claim they had not rejected but were "studying" Google Voice, which Google contradicted. The biggest surprise in the whole scenario is that AT&T said that they had no problem with Google Voice on the iPhone. Months later, there's still no official Google Voice app in the App Store. You can view the long version of the story here.
The mobile web version announced today (iPhone 3.0 required), while not the first, utilizes HTML 5 to accomplish some new and impressive tasks. For example, it lets you display your Google Voice number on outgoing calls and provides easy access to voice mail plus text messaging (send and receive). Additionally, you can dial with the virtual keypad and read transcripts of messages.
It's not all roses, of course. For instance, it won't access your iPhone's contact list or push incoming SMS.
Unless they're going to limit the capabilities of the iPhone's browser, or somehow block access, there isn't much Apple can do to prevent users from trying it out.
Note that this is limited to the US. To try it out, point Mobile Safari to http://m.google.com/voice. Pro tip: Add a bookmark to your iPhone's home screen to launch with a tap.