I think, at least something has changed with version 0.9.9.21, but it still doesn't work. Consider the following code:
{%value=poster maxvalues="1" width="50" height="1000"}
Any poster wider then 50 pixels should be shrinked to 50 pixels in width while preserving aspect ratio. Actual behaviour it is quite strange, because now (if it's smaller than 1000 pixels in height) it is going to be stretched to 1000 pixels in height while preserving aspect ratio. So it gets even wider than it was originally.