This attack has not yet been confirmed as terrorist related, but ISIS did promise London (and Washington D.C.) would be next. A "Somali Norwegian" youth suspect (he is arrested) who is accused of randomly attacking strangers and ISIS celebrating certainly point to a Islamist connection--even if it turns out to be a mentally unstable person inspired by ISIS. There was a similar attack by a Somali man in London last year.
One of the stated goals for ISIS is to disrupt tourism in Europe. ISIS makes clear it wants to dominate Europe by 2020.
Instapundit: Five stabbed, one dead in London
List of Islamist Terror Attacks (last 30 days)
Lem's Place: Did ISIS build a secret network of killers?
Small Dead Animals: Loony Wolf
Weasel Zippers: Is Monty Python advising anti terrorist techniques: Run Away
Victory Girls: Mental issues "triggered" Somali man to go on knife wielding rampage and kill
American Power Blog: Don't dare blame Islamist Terror for this... and Daily Mail says it was over being bullied in school?
Blazing Cat Fur: German may face charges for insulting Munich Muslim Massacre Shooter
AoSHQ: Ace has a theory on motives for this attack
Pam Geller: Devote Muslim? Does being devote Muslim = Mental Illness?
Twitchy: Nice but mentally ill London attacker studying jihad?