Threads has (as of late last week) enabled federation for a small set of employee accounts. Here's a federated post; it doesn't look like comments and likes are federating: https://mastodon.world/@mosseri@threads.net/1115866154367492...

I don't think most instance owners will block them. The larger servers I've paid attention to don't plan to at this time, though many people have expressed concerns about it being an attempt by Meta to embrace, extend, and extinguish the Fediverse. A few people are very loud about not wanting a Meta presence and are trying to convince others to block them.

I think the opportunity outweighs the threat. There's a chance for Mastodon and others to show their software to a mainstream audience which has already shown a willingness to try out new social software by joining threads. Ideally, some projects will have a better pitch than just "we're not corporate".