Bug#490704: Some further info
I had this exakt problem on a brand new T61p using lenny and the 2.6.25-2 amd64 kernel. It happened when I transfered a 1G Mail directoty over my wireless connection using ssh. Logging into the frozen machine from another computer didn't work.
I tried reproducing the effect doing the same transfer, but this time the transfer succeeded.
The card I'm using is a 4965AG, but searching the net, this seems to be a common GNU/Linux problem for people with different wireless cards.