Not a question, more a 'results of empirical experiment'...
HTC Desire, Dell Optiplex G620, Ubuntu 11.04 Unity + Gnome
The above combination does not mount the Desire if any of the following are true:
1. You use the Front USB Ports
2. You use a Rear USB port for a Cherry keyboard (RS14100 USB/FPR)with a USB Hub and you then plug the phone into the hub.
I suspect that it is to do with the USB power because if you connect the phone to the rear port directly it mounts fine.
Err, now I feel a little puzzled because having plugged the thing in on the rear and successfully mounted it, using the front ports now seems to work as expected.
Interestingly if you use the same phone in the front ports on a Dell Precision 380, Ubuntu 11.04 + Xfce everything behaves as you would expect.
On the GX620 Ubuntu 10.10 + Gnome both the failing port combinations worked.
/me heads down to the shops for a long micro USB cable
dmesg tail for when it fails on the GX620 (Keyboard Hub)
[1200560.921104] usb 4-2.1: new full speed USB device using uhci_hcd and address 13
[1200561.023096] usb 4-2.1: not running at top speed; connect to a high speed hub
[1200561.051284] usb 4-2.1: rejected 1 configuration due to insufficient available bus power
[1200561.051291] usb 4-2.1: no configuration chosen from 1 choice
dmesg tail for when it works on the GX620
[1199663.220020] usb 1-5: new high speed USB device using ehci_hcd and address 4
[1199663.366614] scsi5 : usb-storage 1-5:1.0
[1199664.367168] scsi 5:0:0:0: Direct-Access HTC Android Phone 0100 PQ: 0 ANSI: 2
[1199664.368468] sd 5:0:0:0: Attached scsi generic sg1 type 0
[1199664.375515] sd 5:0:0:0: [sdb] Attached SCSI removable disk
[1199693.012816] sd 5:0:0:0: [sdb] 7698432 512-byte logical blocks: (3.94 GB/3.67 GiB)
[1199693.017046] sd 5:0:0:0: [sdb] Assuming drive cache: write through
[1199693.025778] sd 5:0:0:0: [sdb] Assuming drive cache: write through
[1199693.186722] sdb: sdb1
[1199736.039651] sdb: detected capacity change from 3941597184 to 0
[1199745.199561] usb 1-5: USB disconnect, address 4