I am also experiencing the same problem.
I have a Debian 6 box with a:
Bus 001 Device 003: ID 148f:3070 Ralink Technology, Corp. RT2870/RT3070 Wireless Adapter
And compat-wireless (2x00usb) version 3.2-rc1-1.
I have set up hostapd v1.0 with the following config:
The AP works like a charm sometimes but others it will not even log the association requests. The change seems to happen when I restart hostapd service, but in order to restore it I have to restart it many times.
The problem seems to be random, but I have made some wireless captures with airpcap and it seems to be that when the AP does not allow associations no probe responses are sent from it upon probe requests from the client.
I am attaching the captures along with the hostap log the captures I made to be read with wireshark.
The problem also ocurred with hostapd v0.6 (the one included in the repositories of Debian 6) so I am inclined to think the problem is the driver.
I am willing to help any way I can testing patches or, with some guidance, looking at the code.