I know the line is blurred between front end and back end developers, but at what point (most of the time) does my project go from MAYBE requiring a back end developer, to definitely needing one. Are there certain features....User Registration? Database? CMS?
It can vary, obviously, but often designers will be capable of doing basic front-end development. On the other hand, any back-end developer worth their salt will be able to do the front-end as well, provided they have a design to work with.
If all you need is the front-end, ie, the static pages, then no, you likely don't need any work done on the back-end. If you do need any type of CMS or anything custom that relies on a database, then yes, you will need a developer capable of working on the back-end, or at least one familiar with your CMS of choice. When looking for a developer, save yourself some time and go ahead and get one who can do both.
I recommend separating the design and development pieces. Have a designer create a design for you, and then get a developer to cut it and skin your website. The majority of the time, when dealing with individuals versus a firm or group, this will get you a better result. The exception may be if your site is static and fairly simple, the designer may be fully capable of going ahead and doing everything for you.
Backend developers are usually people who do heavy database stuff, build service oriented servers, program EJBs or EAI servers. If you want to cluster something, you probably need an Backend engineer.
But the separation is not really clear. A frontend developer sometimes programs f.e. in PHP and operates on server side, doing sql selects or builds up some services for ajax requests. This is backend programming, but most people can do it.
I think you should go with a backend engineer if you plan with clustered NoSQL or relational databases, integrating several servers together or something like that.
If you program with PHP and use a cloud announced as project as a service, you probably need not an expert in backend programming.