Am I able to lock the screen into portrait mode?

[Mokonoko]

I like using my phone on my bed or lying down but it gets a bit annoying how the screen switches to landscape when I turn to the side(which I often do)

 

[Winterfang]

I don't think you can, at least not regularly.

 

[jfa1]

Not at the present but it may be coming with Windows 8 Phone

 

[rbrunner]

Only for unlocked phones

There is an app, but unfortunately only for developer-unlocked phones. If yours isn't unlocked, you will not be able to (side-)load the app:

Orientation Lock Release - Windows Phone Hacker

Thinking about it, that would be a really nice feature for WP7.8 - for Microsoft, rather easy to implement as a basic new OS feature.