Noticed that on my Arduino Pro Mini it works without the cap because apparently that one has the auto-reset out-of-the-box.
On the Arduino Uno it didn't. As it only worked if I pressed reset right before the sketch was uploaded (or when using the 1?F cap).
john1993:
most arduinos already have that circuit in place so that might be the problem. for stand alone chips 10k pullup to vcc is also normally used but i prefer a diode there instead. there are sometimes issues with voltage mismatch between dtr and reset pin which this solves for every setup ive encountered.ps type of cap dont matter.