DHCP Relay And Switch Port Settings?

I am actually more of a 3Com guy than Cisco but I like to think I know my stuff and having just discovered this site I hope to join in as it looks a good place.I have a question about the general theroy underneath DHCP relay (IP Helper in the Cisco world I believe!)I've always put the DHCP server in VLAN1 only. A few weeks ago, I repatched the DHCP server in one of our offices (small site, single switch) into a different port by mistake. A few days ago I noticed (when doing something entirely unrelated) that I had left it in a port on both VLANs meant for PC's/phones. So, untagged on VLAN1, tagged on the voice VLAN. It had continued to work properly all that time however.By contrast, at a larger site, a new DHCP server came online and was put on a switch elsewhere in the building and was also put into a port on both VLANs - however things didn't work at all - of course we noticed immediately and changed the port setting to VLAN1 only, and it came back.In the second example, the server was on a different switch to where the main L3 switch for the Voice VLAN was (which also does the DHCP relay), and was connected over a fibre trunk link.Is there any reason why it should work on a single switch on a port on both VLANs, but not when connected over a trunk link? The server does not understand tagging. In the case where it worked, the phones were getting an IP from the correct scope; so the relay obviously must have been working with the server issuing an address based on the GIADDR field.