100% power or network uptime.. ? It's difficult to achieve. Many webhost do claim about 100% uptime but we should go through their TOS/SLA before talking such host into consideration.
It is difficult but not unheard of from some providers but 99.9% is more of an industry standard. Cloud will allow you to have 100% uptime without a doubt.
Mostly it is possible with cloudhosting, but there are some very good data centers who provide uptime almost very close to 100%, like softlayer is one of the top end provider, their uptime records are very good though.
100% Uptime never includes maintence mode or hardward upgrade. Also, we usually guarantee 99,99% Uptime, it means every month datacenter or provider can set downtime to maximum 4 hours.