I am currently using a dotted border for certain UI stuff such as instructions, notes, error boxes, etc.
But recently I changed to a solid border, due to a requirement, but I just find it kind of strange.
It seems that by making it solid it puts too much emphasis on page elements which are just informational.
What are your views?
This is a difficult question because it is very subjective. Many newer websites these days (Stack Overflow included) fore go the border completely and use a more subtle background color difference between the main body color for accented content sections like you mentioned (instructions, notes, etc.). When I see dotted lines, I intrinsically relate them to abbreviations or acronyms like this:
<abbr style="border-bottom:1px dotted #000;cursor:help;">
<span style="cursor:help;" title="Cascading Style Sheets">CSS</span>
</abbr>
Therefore I would recommend staying away from dotted borders.
The point of contention here, I believe, is contrast. Your motive in using a dotted border is to reduct contrast vis-a-vis the background visual element containing it.
You can achieve the same effect using a muted color (a shade of light grey on a light-colored background, for example) so that its emphasis will not be different from using a dotted border.
You might also want to circumvent the requirement by using a thinner border width, if that is possible.
BTW: What interface are we talking about? Windows or web?
I use dotted borders to mean, "you can drag and drop something on top of this area". Also known as a Drop Target.
For emphasis, I use color, bold, or background color. Borders will sometimes be confused with "clickable" so be careful with them.