One of the best ways to allow other devices to use your phone’s data plan to get on the internet (a process called tethering) is to have your phone become a little wifi hotspot. Then, any wifi device can get internet access via your phone. To do this with the HTC Aria, follow these steps:

1. Root the phone using Unrevoked
2. Install the latest version of android-wifi-tether (I tested 2.0.5-pre4) found here: http://code.google.com/p/android-wifi-tether/downloads/list. You may have to use the Android SDK or Sideload-Wonder-Machine to install the software.
3. Find the proper firmware (fw_bcm4329_ap.bin located in the /system/etc/firmware directory of a HTC Evo 4g system-dump) and rename it to fw_bcm4329.bin. Place this file in your android.tether directory on your SD card. The directions I followed were here:http://code.google.com/p/android-wifi-tether/wiki/Setup_HTC_Desire_Incredible
   A few links I found were:
   http://www.htcevoforum.net/f2/htc-evo-system-dump-18/
   http://rs922.rapidshare.com/files/389762848/system.rar

If things don’t work out, check the android-wifi-tether log to make sure it has found and is loading the firmware file.
I have confirmed that this procedure works for unencrypted wifi tethering on my HTC Aria to a Ubuntu laptop. You can control access via MAC address filtering, but keep in mind that anybody with a wifi card can see all of your network traffic (and MAC address spoofing is trivial.)

I am currently attempting to get WEP-128 bit encryption working perfectly with Ubuntu Linux. (Wifi-Tethering does not currently support WPA encryption.) I have been able to get the phone to export a WEP encrypted wifi signal, but the Ubuntu Network manager has a problem connecting to it. I am able to connect to the WEP encrypted wifi form the phone if I configure the laptop’s wireless interface manually with a script such as the following:

sudo service network-manager stop
sleep 1
sudo ifconfig eth1 down
sudo iwconfig eth1 mode ad-hoc
sudo iwconfig eth1 essid AndroidTether
sudo iwconfig eth1 key s:SUPERSECRETKEY
sudo ifconfig eth1 up
sleep 2
sudo dhclient eth1