yeah unfortunately you do need a release.. i think.
I know people will rip me to shreds for saying this, but if you know him or her well, and you send the photo in an email asking "can I use this in my portfolio" and they reply "yes". I wouldn't sweat over it. You have their permission in writing. My best friend, and some family are in my portfolio and they didn't sign releases.
If it's a stranger, you better get a signature and maybe even a witness. If you want to use the picture in anything other than an online portfolio, you need a contract, (even for family). If they hired you, you need an agreement that you can use the picture.
I don't know the laws solid. See, I'll go to a thanksgiving party at a friends, take pictures of everything including strangers and then upload them to facebook, that's allowed? But, as I understand, if their in your portfolio, then it's considered "advertising" (for yourself or your company) and a release should be required.
Just be smart about it. I was in a situation once where I assumed, "they won't care" and they did. Luckily I didn't do anything stupid from there. I've also had a nude shoot before where someone signed a release and a year or so later asked me to take the photo down, which I did. Respect what they want, reputation is very important, and always be upfront about what you plan to use the photos for before shoots. Never settle with verbal agreements, write out an agreement on a napkin if you must.