It varies greatly on the project you're working on. There are people out there trying to automate account creation on web based email's for the sole purpose of email spamming (same with social networks). And with complex botnet proxies and the TOR project, false accounts are getting harder to detect.
They have created interesting methods to combat Captcha images. They even have outsourced OCR services, where people in other countries are paid to solve ocr 1 image at a time. [0]
Some other things to worry about, if you sell anything on ebay or amazon as a hobby. They have pretty complex systems to detect linked accounts. If someone was to log into a "banned seller" account on your network. It can be a nightmare to convince ebay or amazon that it wasn't you. and you can most likely be banned on their systems forever (to sell). Just seems like a lot more to worry about.
They have created interesting methods to combat Captcha images. They even have outsourced OCR services, where people in other countries are paid to solve ocr 1 image at a time. [0]
[0] http://deathbycaptcha.com