Dialup is not static, you generally get a new one everytime you dial up.
Nearly every ISP refreshes your address every 24/48 hours.
If you've a home network, the only device in your house that has an IP address actively on the internet is your router.
The router than gives everything connected to it an IP address (this is called subnetting) and acts as a go-between from the internet to you and vice versa.
My point is that while yes, it may be possible for Google to investiage click fraud, there's also a lot of valid reasons why a number of people would visit the same website from the same IP.
[Edit for clarification]
Keep in mind static IPs are rare and generally only owned by ISPs and the likes. Your ISP can issue you a static IP from their end, for sure. But its not an "offical" static IP as registered with ICANN and what not. Again, its just subnetting.
The only permanent address all network cards have is their MAC address. Cellphones also have ESNs, other devices similar things.
Each packet of information you send and receive has your MAC address and this is used by routers to know where to send your requests for stuff on the internet back to you.
There, now my networking certification wasn't entirely a waste. 