It seems that i've identified the problem as actually being a codec problem.
I looked a little closer at the verbose logs in asterisk and saw messages like this:
[Nov 19 02:57:44] WARNING chan_gtalk.c: Asked to transmit frame type alaw, while native formats is 0x4 (ulaw) (read/write = ulaw/ulaw)
It seems that whenever I was making calls, the formats kept on flipping from ulaw to alaw and vice versa. When it did work, only one end could hear the other. To make matters worse, I couldn't get G722 to work whenever I tried to force it.
Comparing the OpenWRT Asterisk 1.8.x install to a standard install on Ubuntu, it looks like the codec files for G722 weren't even getting installed. I modified the packaging and republished additional packages at my repository for Asterisk on arctangent.net (http://arctangent.net/~superm1/openwrt)
Now there is two more packages available, one for alaw and the other for g722. You should be able to update your opkg list and install them:
# opkg install asterisk18-codec-alaw asterisk18-codec-g722
After they're installed, modify sip.conf to make sure you explicitly mention that you now support G722:
Now restart Asterisk and Sipdroid should be able to use G722 for some much better audio.