This was driving me crazy all last night and this morning, thinking I had some rogue bug in my code but it turns out it is impossible to set an item's position if the item is < 10ms long?
None of the pre-existing extension actions that move an item work if the item is that short, and this simple Python script does absolutely nothing unless the item is > 10ms:
item = RPR_GetSelectedMediaItem(0,0);
RPR_SetMediaItemInfo_Value(item, "D_POSITION", 500);
http://www.adamwathan.com/reaper/APIhell.swf
Yikes!
This is at least a problem when developing in C++ and Python, using SetMediaItemInfo_Value or GetSetMediaItemInfo.