NETDEV WATCHDOG: eth0: transmit timed out - what does this mean?

I recently switched my gentoo box on and was unable to access the
network. I was unable to fix the problem until I rebooted. Its been
running OK for almost 24h since I rebooted now and there aren't any
similar messages in the logs. I discovered the following in my
/var/log/messages. What does it mean?

Apr 26 22:29:29 basement NETDEV WATCHDOG: eth0: transmit timed out
Apr 26 22:29:29 basement eth0: Transmit timeout, status 0d 0000 c07f
media 10.
Apr 26 22:29:29 basement eth0: Tx queue start entry 4 dirty entry 0.
Apr 26 22:29:29 basement eth0: Tx descriptor 0 is 0008003c. (queue
head)
Apr 26 22:29:29 basement eth0: Tx descriptor 1 is 0008003c.
Apr 26 22:29:29 basement eth0: Tx descriptor 2 is 0008003c.
Apr 26 22:29:29 basement eth0: Tx descriptor 3 is 0008003c.
Apr 26 22:29:29 basement eth0: link up, 100Mbps, full-duplex, lpa
0x45E1
Apr 26 22:29:41 basement NETDEV WATCHDOG: eth0: transmit timed out

My gentoo machine is an unattended server and I am concerned that this
might happen at an inopportune time. I have recently upgraded to
gentoo-sources-2.6.11-gentoo-r6. The relevant output of lspci (if
this is of any help) is:

0000:01:0b.0 Ethernet controller: Realtek Semiconductor Co., Ltd.
RTL-8139/8139C/8139C+ (rev 10)

Could it be a kernel or a hardware problem?