Large integrations have worked at times in the past mostly when one company bought a competitor (or semi-competitor) for its market share. Generally they haven't been great for employees of the company that was taken over, though.
Some examples include: DEC/Digital (bought by Compaq), Netscape Server business (bought by Sun at the same time AOL bought the rest of the company), Compaq (bought by HP), Lotus (bought by IBM in a hostile takeover), Peoplesoft (hostile takeover by Oracle), BEA System (bought by Oracle), Sun (bought by Oracle).
There are also numerous examples in the telecommunications market.
The Oracle/Sun deal is the most interesting in this context, because it is a software company taking over a hardware company (yes, Sun has software, but one of the reasons Oracle bought it was to compete with HP & IBM on complete system integration all the way down to hardware).
It's clear there are plenty of examples where this didn't work. Google has a huge challenge, but there are some precedents for this working.