As usual AppSync will be available  for iOS 4.1 as soon as apple release the public version of iOS 4.1. So if you jailbreak your device you’ll be able to install Appsync via Cydia then install cracked ipa files by iTunes (it’s working with iTunes 10). -How To?-

But the problem is in jailbreaking newer devices. As one knows the newer iDevices (including iPhone 3GS, iPhone 4, iPad & iPodTouch 3G & 4G) can’t be jailbroken by jailbreaking tools that uses pwnexploit such as PwnageTool, redsnow, etc. Note that there is a huge diffrence between pwn tools (redsnow,etc) and userland jailbreak tools (Spirit, Jailbreakme).

So You should wait until someone find an exploit or a hole in iOS 4.1 to jailbreak it. I think it will happen soon.