I am not an IP lawyer, but seems like copyright infringement to me. Starting with the name. I don't think it matters if twitvid.io is not a .com - if they are providing services under the twitvid mark, they own it. The design elements might be web 2.0/twitterish, but the placement on the screen and wording screams knock-off. It is twitvid.com who should change the name, if .io guys were indeed to market first. It will be interesting to see if the .io guys take this to court.

Having said that, I agree with the comments about making sure to grab a .com. If anything, to avoid a situation like this. Does anyone know, did del.icio.us buy delicious.com right away, or after they became popular and got some cash in their pockets?