Well, unless you are one of the (fairly rare) people who have multiple internet-facing IPs in the location that they do their personal surfing from, rather than just a NAT box, your multiple computers won't do all that much. Even if you don't shell out for a static IP, most home broadband IPs are, de facto, stable for a few days at a time, if not rather longer. Multiple distinct signatures aren't a huge mystery if they come from the same IP.
Unless you are quite careful, multiple browsers is trivially defeated by Flash cookies, which are persistent per flash instance, not per browser(maybe Chrome's upcoming integration will change this, I don't know). The other plugin and font fingerprinting stuff should be reasonably robust cross-browser as well.
Then there are the time-of-day based inferences. IP geolocation should, barring specific attempts at obfuscation, or the occasional fuckup, at least get you within the right time zone. You can then start testing inferences based on the fact that, for instance, schoolchildren tend to browse at home earlier than office workers do, night-shift workers have a different schedule altogether, stay-at-home-moms keep roughly the same hours as work-from-home consultant types; but have different browsing habits, and so forth.
I'm not saying that privacy is completely impossible, just that it is harder than it looks.