While you're reading this, hundreds of people are hard at work on getting Ice Cream Sandwich onto current Android hardware, long before manufacturers and carriers will be sending out official updates. We've covered some of the earliest and most interesting ICS projects on this very site, but there's just too many to cover every single one. Luckily, we don't have to: a friendly member of the XDA-Developers forum has put together a list of all the active Ice Cream Sandwich ports on the prolific modder site.
Pretty much every major device released in the last 18 months, and many many more, are represented. The ports themselves are at various stages of completion - naturally the options for the Nexus One, Nexus S, OG DROID and the like will be the most feature-complete, while newer and less popular phones and tablets will get the short end. Note that the list only extends to XDA, and if your device isn't on there it may be getting love somewhere else, like RootzWiki. Some of the builds are straight from Ice Cream Sandwich's source code, while others are based on CyanogenMod 9.
As always, be wary when trying out new ROMs. That's doubly true with a brand new operating system, which still requires a lot of kinks and bugs to be worked out. Keep a Nandroid backup of your "daily driver" account, kiss your warranty goodbye, and have fun. If you're a Nexus S user, be sure to check out our hands-on look at a very early version of CyanogenMod 9.