I also experienced this problem a few times. Running 10.04 LTS here. At some point my wired network connection just suddenly stops working, restarting networking, using Network Manager to connect/disconnect, or physically removing/inserting the cable doesn't help, I have to reboot the whole machine. The leds on the NIC are flashing though, indicating a connection.

Linux lpt-marc 2.6.32-25-generic #45-Ubuntu SMP Sat Oct 16 19:52:42 UTC 2010 x86_64 GNU/Linux

# cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=10.04
DISTRIB_CODENAME=lucid
DISTRIB_DESCRIPTION="Ubuntu 10.04.1 LTS"

From the /var/log/messages:

Oct 27 12:56:46 lpt-marc kernel: [431343.040063] ------------[ cut here ]------------
Oct 27 12:56:46 lpt-marc kernel: [431343.040084] WARNING: at /build/buildd/linux-2.6.32/net/sched/sch_generic.c:261 dev_watchdog+0x262/0x270()
Oct 27 12:56:46 lpt-marc kernel: [431343.040091] Hardware name: Latitude D830                   
Oct 27 12:56:46 lpt-marc kernel: [431343.040097] NETDEV WATCHDOG: eth0 (tg3): transmit queue 0 timed out
Oct 27 12:56:46 lpt-marc kernel: [431343.040102] Modules linked in: nls_iso8859_1 nls_cp437 vfat fat binfmt_misc vboxnetadp vboxnetflt vboxdrv vmnet vmblock vsock vmci vmmon ipt_MASQUERADE iptable_nat bridge stp kvm_intel kvm ipt_REJECT xt_comment ipt_LOG xt_limit xt_tcpudp ipt_addrtype xt_state pcmcia usbhid dell_wmi ppdev arc4 ip6table_filter snd_hda_codec_idt ip6_tables nf_nat_irc nf_conntrack_irc nf_nat_ftp nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_conntrack_ftp nf_conntrack iwl3945 iptable_filter iwlcore ip_tables joydev x_tables mac80211 led_class usb_storage parport_pc snd_hda_intel snd_hda_codec snd_hwdep snd_pcm_oss snd_mixer_oss snd_pcm snd_seq_dummy snd_seq_oss snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq hid snd_timer snd_seq_device dell_laptop video dcdbas nvidia(P) output fbcon tileblit font yenta_socket rsrc_nonstatic pcmcia_core snd bitblit softcursor cfg80211 psmouse serio_raw intel_agp vga16fb vgastate soundcore snd_page_alloc lp parport ohci1394 ieee1394 tg3
Oct 27 12:56:46 lpt-marc kernel: [431343.040283] Pid: 0, comm: swapper Tainted: P           2.6.32-25-generic #45-Ubuntu
Oct 27 12:56:46 lpt-marc kernel: [431343.040289] Call Trace:
Oct 27 12:56:46 lpt-marc kernel: [431343.040294]  <IRQ>  [<ffffffff81065efb>] warn_slowpath_common+0x7b/0xc0
Oct 27 12:56:46 lpt-marc kernel: [431343.040315]  [<ffffffff81065fa1>] warn_slowpath_fmt+0x41/0x50
Oct 27 12:56:46 lpt-marc kernel: [431343.040324]  [<ffffffff81478b92>] dev_watchdog+0x262/0x270
Oct 27 12:56:46 lpt-marc kernel: [431343.040334]  [<ffffffff8107fd27>] ? insert_work+0x77/0xc0
Oct 27 12:56:46 lpt-marc kernel: [431343.040345]  [<ffffffff810387b9>] ? default_spin_lock_flags+0x9/0x10
Oct 27 12:56:46 lpt-marc kernel: [431343.040354]  [<ffffffff81478930>] ? dev_watchdog+0x0/0x270
Oct 27 12:56:46 lpt-marc kernel: [431343.040362]  [<ffffffff8107688b>] run_timer_softirq+0x19b/0x340
Oct 27 12:56:46 lpt-marc kernel: [431343.040373]  [<ffffffff8106d587>] __do_softirq+0xb7/0x1e0
Oct 27 12:56:46 lpt-marc kernel: [431343.040381]  [<ffffffff810c4540>] ? handle_IRQ_event+0x60/0x170
Oct 27 12:56:46 lpt-marc kernel: [431343.040391]  [<ffffffff810132ec>] call_softirq+0x1c/0x30
Oct 27 12:56:46 lpt-marc kernel: [431343.040398]  [<ffffffff81014cb5>] do_softirq+0x65/0xa0
Oct 27 12:56:46 lpt-marc kernel: [431343.040406]  [<ffffffff8106d425>] irq_exit+0x85/0x90
Oct 27 12:56:46 lpt-marc kernel: [431343.040414]  [<ffffffff81548a65>] do_IRQ+0x75/0xf0
Oct 27 12:56:46 lpt-marc kernel: [431343.040422]  [<ffffffff81012b13>] ret_from_intr+0x0/0x11
Oct 27 12:56:46 lpt-marc kernel: [431343.040426]  <EOI>  [<ffffffff8130ec1d>] ? acpi_idle_enter_bm+0x294/0x2c8
Oct 27 12:56:46 lpt-marc kernel: [431343.040444]  [<ffffffff8130ec16>] ? acpi_idle_enter_bm+0x28d/0x2c8
Oct 27 12:56:46 lpt-marc kernel: [431343.040455]  [<ffffffff81439907>] ? cpuidle_idle_call+0xa7/0x140
Oct 27 12:56:46 lpt-marc kernel: [431343.040464]  [<ffffffff81010e73>] ? cpu_idle+0xb3/0x110
Oct 27 12:56:46 lpt-marc kernel: [431343.040472]  [<ffffffff8152e72b>] ? rest_init+0x6b/0x80
Oct 27 12:56:46 lpt-marc kernel: [431343.040482]  [<ffffffff8184bdcc>] ? start_kernel+0x368/0x371
Oct 27 12:56:46 lpt-marc kernel: [431343.040490]  [<ffffffff8184b33a>] ? x86_64_start_reservations+0x125/0x129
Oct 27 12:56:46 lpt-marc kernel: [431343.040498]  [<ffffffff8184b438>] ? x86_64_start_kernel+0xfa/0x109
Oct 27 12:56:46 lpt-marc kernel: [431343.040505] ---[ end trace a25f9e500e8fa2dc ]---
Oct 27 12:56:46 lpt-marc kernel: [431343.268176] tg3: eth0: Link is down.
Oct 27 12:56:49 lpt-marc kernel: [431346.375485] tg3: eth0: Link is up at 1000 Mbps, full duplex.
Oct 27 12:56:49 lpt-marc kernel: [431346.375492] tg3: eth0: Flow control is on for TX and on for RX.