It is safe because you won't get a brick, but you will lose your jailbreak. Currently, the only way to jailbreak an iPad on 4.2 is Redsn0w, and it is tethered (you need to rejailbreak everytime you turn on your iPad, resprings don't count). But apparently there will be a new untethered jailbreak before Christmas. If you update while you have your SHSH blobs backed up, you can always just downgrade (good if you just want to try 4.2).
Can't find any threads about this particular issue on GBAtemp or the wider intarwebz. Saw one video on YouTube that seemed relevant, but it was some dude with the same issue, no solutions offered in the comments. :/