My laptop is connected to the internet by ethernet cable, and in Ubuntu I was able to setup a wireless network that my phone could connect to (I think it was Network Settings, Wireless, then there was s Wireless Hotspot button).
In Xubuntu, I go: Network Connections, Wireless Add, set it up as "Ad Hoc" and then "Shared to other computers", but it offers no option of WPA (and my phone's having difficulty connecting to the WEP). Which I think was the security offered in the Ubuntu Ad-hoc network.
How do I get WPA in Xubuntu?
The kind of security options (X)Ubuntu makes available for setting up an ad-hoc network depend on your wireless card. Some support WEP-only, others WPA and others none.
Ubuntu relies on the requirements relayed by the wireless card driver, and there's no way to force an "unsupported" method without modifying the network-manager source code.
If you are having problems connecting with WEP, please try no security.