1) If I yank a line with yy, pressing P or p will insert the text into a new line.
2) If I go to visual mode and select a region, yank it with y, then press p/P, it gets it inserted into the current line.
That is, vim preserves new lines.
What I can't figure out is how to paste into the current line in the first case, and, vice-versa, how to easily paste into a new line in the second case (without doing :put, or opening a new line).