Is it possible to use VoIP over gprs from an Android phone? Either using a SIP client or Skype?
There a quite a few SIP clients last time I looked, but you need some sort of ID you have to purchase elsewhere to make use of them. Skype is supposedly coming soon and I think is available right now for Verizon Android phones.
Ditto Fring. Its a great app and probably one of the easier ones to use. Plus you can do video conferencing with Fring.
There are a number of sip clients available such as Sipdroid, CSipSimple, and Linphone. Sipdroid includes an option to create a pbxes.org account linked to your Google Voice account, allowing you to make and receive calls to and from PSTN numbers for free, assuming you are in the US. (After the account is created, any sip client can be used with the pbxes.org account.)
If you want to setup your own Asterisk server, you can configure the trunking with Google Voice yourself. Or you can use any other VOIP service that provides SIP/PSTN trunking.
An alternative option is GrooVe IP, which is a Google Talk client with voice support. This uses XMPP/Jingle directly between your android device and Google Talk/Voice. The free version only supports WLAN networking; the paid version works over mobile networks.
Try Fring it is all in one, Unfortunately they do not support Skype anymore, But ther is SIP, Gmail, MSN and many others.