When you place a widget on the desktop, first of all, it doesn't always go where you drag it to, sometimes it appears somewhere completely different. Then, most of the time, if you try to drag it to another location, it simply won't move.
Blocks: (none) => 17523
In current Cauldron/Plasma you can drag the widgets around (usually) by clicking near their top.
Status: NEW => RESOLVEDResolution: (none) => FIXED