Yes, its true! If you want untethered iOS 4.3 jailbreak on your iDevice in future, you should save iOS 4.2.1 SHSH blobs now. MuscleNerd, who happens to be one of the core members of iPhone Dev-Team confirmed that his team is working on iOS 4.3 untethered jailbreak which will rely on iOS 4.2.1 SHSH blobs. So if you haven't already saved iOS 4.2.1 SHSH blobs for your iDevice, save it now using the latest version of TinyUmbrella.
@comex and now one goal is to get Monte to untether 4.3-final using 4.2.1 SHSH blobs (and future FW X->FW Y's, etc) [via MN]
To save iOS 4.2.1 SHSH blobs, you don't even need a jailbroken device. Just connect your iDevice running iOS 4.2.1 to your PC via USB cable and hit the Save SHSH button. You can find step-by-step guide to save SHSH blobs using TinyUmbrella here.
If you depend on an unlocked iPhone and want to jailbreak iOS 4.2.1 untethered on iPhone 3GS and iPhone 4 while preserving the baseband, use PwnageTool. If you don't rely on an unlocked device, just follow the GreenPois0n RC5 jailbreak tutorial for Windows and Mac.