Packages that are "not to upgrade" are held back for one reason or another. Typically packages are held back because they have unmet dependencies. For example, you install package foo version 1 and two months later package foo version 2 is released, which now requires package bar (which is not installed) - since package bar is not installed, the dependencies for package foo version 2 aren't met and it is held back.

Using apt-get dist-upgrade is more dangerous - it'll allow apt to pull in packages which are not already installed (which will allow it to satisfy dependencies, when possible, in cases of held back packages). I've done it on my N900, but you have to be very, very, VERY careful what packages you allow it to upgrade/install - especially on Maemo. I've heard stories of people bricking their device due to a dist-upgrade... so proceed with extreme caution if you seriously think about trying this.