Plots of USRP decimation woes: input is GMSK waveform @ ~30ksym/sec, BT = 0.35, no noise added using decimation rate of 16: http://img339.imageshack.us/img339/4467/d16cz9.jpg
top plot is complex baseband, bottom plot is amplitude of top plot some amplitude variations noticeable, but fairly minor.
Plots showing effects of carrier discrepancy between tx and rx on GMSK demodulation: These are plots of the output of the fmdemod block (and of the input to the M&M clock recovery block).
Residual carrier manifests as DC offset after fmdemod (look at y-axis): http://img89.imageshack.us/img89/1116/fmdemoduntunedri8.jpg sending input like this into the clock recovery block -> lots of bit errors, especially when noise is added.
So, that's the motivation for wanting carrier tracking. I think Eric's right that there's a solution involving filters that doesn't require me to leave python-land, but I'm still a bit confused as to how to leverage the IIR block.