While we wait for an official Google+ application to appear for Windows Phone, you might want to consider Jacob Bearce's Google+ alternative to the mobile web. A small embedded version of the social network from Mountain View, it uses the HTML5 mobile version of the site, and some dedicated icon charms, to make for a smooth G+ experience.

Yes there's a danger that this is the first in many "launch a website" apps, but the tweaks to the menu bar, and the fact it delivers something that Google haven't yet made available to Windows Phone, is enough for me to recommend this.