You can still downgrade to 4.2.6
I have a Verizon Iphone 4.
Jailbroken using Greenpoison. Then sadly made the same mistake as you all this past Saturday and upgraded to 4.2.7
As of this past Sunday, 4/24/11, I was successfully able to downgrade to 4.2.6 (WITHOUT SAVING MY SHSH Blobs) by using RedSn0w's 4.3.2 jailbreak and the 4.2.6 ipsw and having a tethered jailbroken phone (which sucks compared to non-tethered) but better than no jailbreak.
do a google search for 4.2.6 ipsw. I used the page from sinfuliphone's web page (I'm not allowed to post links here)
Here's the thing though. While installing it the first time around, it kept freezing when trying to install, and I assumed I was out of luck.
If using Windows 7, make sure to FIRST right-click RedSn0w and choose properties. Make sure the box marked "work with Windows Service Pack 3" is checked. After I did that, worked without a hitch.
I then used PkgBackup to restore all my jailbreak programs, settings and folders and I'm almost where I was before all this mess started.