The startup I'm working on now sprung from my senior design project. In our case, our "customer" was a local startup incubator. Their goal was not only to mentor the project, but help us turn it into a business (so retaining ownership of it was critical). We investigated the IP stuff from the outset. After some informal conversations at our school and others, we determined that they mostly don't care about undergrad projects, and we can do whatever we wanted with it.

Most research universities have IP and commercialization people (our school included), but the purpose of that activity is limited mostly to faculty and grad student projects since those projects often require some non-trivial resources from the university (so they want some ROI). Nearly every undergrad project that a student might claim ownership of doesn't require any school resources beyond the standard stuff (maybe you used lab computers to host your site or code repository for a little while during the semester -- not a big deal).

I think in your situation, since you just want to use it as a portfolio piece, I agree with noahc: just post it, but take it down if asked. If you wanted to use the IP for something that might create significant value (read: you'd make enough to make it worth it for the school or the "customer" to sue), I'd ask first and get something in writing (with the help of lawyer, if possible).