Since I upgraded to Lucid 2 days ago I'm struck by this error, too. Never had any problems with karmic, though.
Symptoms are the same: Network gets unresponsive under heavy load. 

Aug 29 16:42:50 COMPAKT kernel: [  404.000051] ------------[ cut here ]------------
Aug 29 16:42:50 COMPAKT kernel: [  404.000078] WARNING: at /build/buildd/linux-2.6.32/net/sched/sch_generic.c:261 dev_watchdog+0x1fe/0x210()
Aug 29 16:42:50 COMPAKT kernel: [  404.000087] Hardware name: Evo N620c (DC824AV)             
Aug 29 16:42:50 COMPAKT kernel: [  404.000095] NETDEV WATCHDOG: eth0 (tg3): transmit queue 0 timed out
Aug 29 16:42:50 COMPAKT kernel: [  404.000102] Modules linked in: binfmt_misc ppdev nfsd exportfs snd_intel8x0 nfs lockd snd_ac97_codec ac97_bus nfs_acl fbcon tileblit font snd_pcm_oss bitblit softcursor snd_mixer_oss snd_pcm auth_rpcgss vga16fb snd_seq_dummy sunrpc snd_seq_oss vgastate snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq snd_timer snd_seq_device joydev snd radeon soundcore ttm drm_kms_helper pcmcia snd_page_alloc drm i2c_algo_bit yenta_socket rsrc_nonstatic pcmcia_core psmouse video serio_raw intel_agp agpgart shpchp output usbhid hid lp parport tg3
Aug 29 16:42:50 COMPAKT kernel: [  404.000239] Pid: 0, comm: swapper Not tainted 2.6.32-24-generic #41-Ubuntu
Aug 29 16:42:50 COMPAKT kernel: [  404.000247] Call Trace:
Aug 29 16:42:50 COMPAKT kernel: [  404.000263]  [<c014c4a2>] warn_slowpath_common+0x72/0xa0
Aug 29 16:42:50 COMPAKT kernel: [  404.000276]  [<c04d996e>] ? dev_watchdog+0x1fe/0x210
Aug 29 16:42:50 COMPAKT kernel: [  404.000286]  [<c04d996e>] ? dev_watchdog+0x1fe/0x210
Aug 29 16:42:50 COMPAKT kernel: [  404.000298]  [<c014c51b>] warn_slowpath_fmt+0x2b/0x30
Aug 29 16:42:50 COMPAKT kernel: [  404.000309]  [<c04d996e>] dev_watchdog+0x1fe/0x210
Aug 29 16:42:50 COMPAKT kernel: [  404.000325]  [<c0163a50>] ? insert_work+0x60/0xb0
Aug 29 16:42:50 COMPAKT kernel: [  404.000340]  [<c012a498>] ? default_spin_lock_flags+0x8/0x10
Aug 29 16:42:50 COMPAKT kernel: [  404.000353]  [<c058cdcf>] ? _spin_lock_irqsave+0x2f/0x50
Aug 29 16:42:50 COMPAKT kernel: [  404.000365]  [<c0163d16>] ? __queue_work+0x36/0x50
Aug 29 16:42:50 COMPAKT kernel: [  404.000376]  [<c015b70e>] run_timer_softirq+0x13e/0x2c0
Aug 29 16:42:50 COMPAKT kernel: [  404.000388]  [<c04d9770>] ? dev_watchdog+0x0/0x210
Aug 29 16:42:50 COMPAKT kernel: [  404.000400]  [<c0153138>] __do_softirq+0x98/0x1b0
Aug 29 16:42:50 COMPAKT kernel: [  404.000411]  [<c012a498>] ? default_spin_lock_flags+0x8/0x10
Aug 29 16:42:50 COMPAKT kernel: [  404.000423]  [<c058cdcf>] ? _spin_lock_irqsave+0x2f/0x50
Aug 29 16:42:50 COMPAKT kernel: [  404.000437]  [<c0106c87>] ? enable_8259A_irq+0x47/0x70
Aug 29 16:42:50 COMPAKT kernel: [  404.000448]  [<c0153295>] do_softirq+0x45/0x50
Aug 29 16:42:50 COMPAKT kernel: [  404.000458]  [<c01533e5>] irq_exit+0x65/0x70
Aug 29 16:42:50 COMPAKT kernel: [  404.000469]  [<c0591245>] do_IRQ+0x55/0xc0
Aug 29 16:42:50 COMPAKT kernel: [  404.000481]  [<c016d5c5>] ? sched_clock_local+0xa5/0x180
Aug 29 16:42:50 COMPAKT kernel: [  404.000493]  [<c0103a30>] common_interrupt+0x30/0x40
Aug 29 16:42:50 COMPAKT kernel: [  404.000504]  [<c016007b>] ? sys_getrusage+0x2b/0x30
Aug 29 16:42:50 COMPAKT kernel: [  404.000517]  [<c039e22b>] ? acpi_idle_enter_simple+0x117/0x148
Aug 29 16:42:50 COMPAKT kernel: [  404.000529]  [<c039df41>] acpi_idle_enter_bm+0xc7/0x29a
Aug 29 16:42:50 COMPAKT kernel: [  404.000543]  [<c04a1e8a>] cpuidle_idle_call+0x7a/0x100
Aug 29 16:42:50 COMPAKT kernel: [  404.000554]  [<c01021d4>] cpu_idle+0x94/0xd0
Aug 29 16:42:50 COMPAKT kernel: [  404.000568]  [<c057a818>] rest_init+0x58/0x60
Aug 29 16:42:50 COMPAKT kernel: [  404.000582]  [<c07a58ec>] start_kernel+0x351/0x357
Aug 29 16:42:50 COMPAKT kernel: [  404.000594]  [<c07a53c7>] ? unknown_bootoption+0x0/0x19e
Aug 29 16:42:50 COMPAKT kernel: [  404.000606]  [<c07a50aa>] i386_start_kernel+0xaa/0xb1
Aug 29 16:42:50 COMPAKT kernel: [  404.000614] ---[ end trace 3b3f35df98f5d5c2 ]---