Does Hashem sit in Shamayim?
Radak: He stands [stationary] - "v'Atah Hashem l'Olam Teshev" (102:13). The same applies to every 'Yeshivah' regarding Hashem - "ha'Yoshvi va'Shamayim" (123:1), "ha'Magbihi Lashaves" (113:5).
Malbim: It is a metaphor, as if He hides His face and His honor is on Shamayim, and He does not oversee. However, since He is Master of the entire land, he mocks [Resha'im].
Why does it say "Yischak", "Yil'ag" and "Yedaber" (verse 5, in the future)?
Rashi: These refer to the present.
Why does Hashem laugh at them?
Radak: They took counsel together to nullify Hashem's word. He sits in Shamayim, hears their words and laughs at them. Dibrah Torah k'Leshon Bnei Adam (the Torah speaks about Hashem as if He were a person, who laughs at one who has no value in his eyes. The same applies to "Hashem Tischak Lamo Til'ag l'Chol Goyim" (59:9), "Hashem Yischak Lo Ki Ra'ah Ki Yavo Yomo" (37:13).
Malbim: This is the one Midah in which the active party differs from the passive. When one is angry, the 'recipient' is punished. When one is merciful, the 'recipient' gets compassion. When one laughs, the 'recipient' is ashamed. This is a metaphor for Resha'im's success. Even though it looks like Hashem laughs and is not angry at them, they are shamed via this. Their punishment increases suddenly when their measure [of sin] is filled - "Boshah Imchem Me'od
