Windows Phone 7.8 is just heading out to consumers, but appears to be wrapped up in a strategic deployment. Should you wish to no longer wait for the update to prolong the lifespan of your Windows Phone 7 handset, XDA Developers forum has the CAB files listed for download. Be sure to file this under "Experienced users only."
As well as Windows Phone 7.8, (build 8858) files are also available for the previous build 8779 aka 'Tango'. So how does one go about this? We published an in-depth tutorial which also builds off of this article on how to go about updating your Windows Phone manually. Both are well worth checking out if you're considering this path. Just like forcing the Tango update, while following such a guide, be sure to ignore previous build numbers in the article and use what's stated over on XDA.
Windows Phone Central cannot take any responsibility should a process you undertake render your Windows Phone useless. Continue at your own peril! And remember, you won't get any OEM specific firmware that may come with the update, so you may want to wait a few days.
Source: XDA Developers; Thanks, tire_007, for the tip