Seems that I too have bricked my 634U. I had Openwgt installed, something went wrong when updating the package list - don't ask me what, but afterwards there was nearly nothing left on the mounted partitions...
Since then, I have cut up two mobile phone data cables (one serial and one USB) and get no response when connecting with ms hyperterm, minicom or screen. I have wired the cables as described on nomis, sarwiki et al., set the parameters to 115200 8N1, no or software flow control - nothing.
In addition, I noticed that there is a voltage of about 3.3 volt on both the RX _and_ the TX pin of the wgt's serial port. Can somebody please confirm whether this is normal? Besides, does anybody have any idea what I might have done wrong and what else I could try to get a serial console?
Grateful for every hint,