Kubuntu doesn't commence booting unless a key is pressed.

On my system, originally a Kubuntu Tribe 3, dist-upgraded to 20070814, I've got the following slightly odd problem:

Upon booting, the pretty "kubuntu" splashscreen with the blue progressbar appears, but only the very leftmost sliver of the progressbar is blue. It stays in this state indefinitely, not moving and not progressing until I press a key, then it immediately springs into action and commences booting.

The machine is an Acer Aspire 5112WLMi, 2xTurion64 TL50 with the following devices as reported by lspci:

00:00.0 Host bridge: ATI Technologies Inc RS480 Host Bridge (rev 10)
00:02.0 PCI bridge: ATI Technologies Inc RS480 PCI-X Root Port
00:04.0 PCI bridge: ATI Technologies Inc RS480 PCI Bridge
00:05.0 PCI bridge: ATI Technologies Inc Unknown device 5a37
00:12.0 IDE interface: ATI Technologies Inc ATI 4379 Serial ATA Controller (rev 80)
00:13.0 USB Controller: ATI Technologies Inc IXP SB400 USB Host Controller (rev 80)
00:13.1 USB Controller: ATI Technologies Inc IXP SB400 USB Host Controller (rev 80)
00:13.2 USB Controller: ATI Technologies Inc IXP SB400 USB2 Host Controller (rev 80)
00:14.0 SMBus: ATI Technologies Inc IXP SB400 SMBus Controller (rev 83)
00:14.1 IDE interface: ATI Technologies Inc Standard Dual Channel PCI IDE Controller ATI (rev 80)
00:14.2 Audio device: ATI Technologies Inc SB450 HDA Audio (rev 01)
00:14.3 ISA bridge: ATI Technologies Inc IXP SB400 PCI-ISA Bridge (rev 80)
00:14.4 PCI bridge: ATI Technologies Inc IXP SB400 PCI-PCI Bridge (rev 80)
00:18.0 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] HyperTransport Technology Configuration
00:18.1 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Address Map
00:18.2 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] DRAM Controller
00:18.3 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Miscellaneous Control
01:00.0 VGA compatible controller: ATI Technologies Inc M56P [Radeon Mobility X1600]
06:00.0 FireWire (IEEE 1394): VIA Technologies, Inc. IEEE 1394 Host Controller (rev c0)
06:01.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8169 Gigabit Ethernet (rev 10)
06:02.0 Ethernet controller: Atheros Communications, Inc. AR5005G 802.11abg NIC (rev 01)
06:04.0 CardBus bridge: ENE Technology Inc CB-712/4 Cardbus Controller (rev 10)
06:04.1 FLASH memory: ENE Technology Inc ENE PCI Memory Stick Card Reader Controller (rev 01)
06:04.2 Generic system peripheral [0805]: ENE Technology Inc ENE PCI Secure Digital Card Reader Controller (rev 01)
06:04.3 FLASH memory: ENE Technology Inc FLASH memory: ENE Technology Inc: (rev 01)
06:04.4 FLASH memory: ENE Technology Inc Unknown device 0551 (rev 01)