AIM is available for most platforms, including Windows Mobile, iOS, and Android, but WP7 is not yet supported, nor have I heard an announcement regarding upcoming support. Unfortunately, many developers are taking a "wait and see" approach before investing time and resources into developing WP7 apps, limiting availability at the moment.
The good news is that you can view the current selection of WP7 apps by downloading the (free) Zune client and then going Marketplace->Apps. That's the same selection you would see browsing available apps directly on the HD7, so you can determine if the apps you desire are currently available or if you should hold off a bit longer.
Hope this helps,
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