I guess, 1 - It implies IT tech related, IO = input/output, as pointed out by otoburb; 2 - Domain name availability, as pointed out by yolesaber; 3 - Google

I guess, 1 - It implies IT tech related, IO = input/output, as pointed out by otoburb; 2 - Domain name availability, as pointed out by yolesaber; 3 - Google named its developer conference 'Google IO', just a guess.