Update: As of 26-Feb-2010, Skype has discontinued both the the Skype Lite and Skype Windows Mobile clients and the download links for both products have been removed from Skype's site.
Although they are no longer available, Skype says that the Skype Lite and Skype for Windows Mobile clients will continue to work at least through the end of 2010. Update: As of 11-May-2012 the Java ME client still works (I don't know if the Windows Mobile one does or not).
I've archived copies of the discontinued clients for your Skyping pleasure:
You can now download the Skype "Lite" Java ME application directly to your phone. Supported phones include most recent Nokia and Sony Ericsson phones and selected Motorola, Samsung and LG models. The site detects your phone model but doesn't stop you from downloading the app with an unsupported phone. Skype Lite is available in signed and unsigned versions. Download Skype Lite
Skype Lite is quite limited compared with other mobile VOIP applications like Gizmo5 and Fring. It uses your mobile voice channel (and minutes) so calls will not be free, even to Skype numbers. In addition, you can only initiate voice calls from Australia, Brazil (Rio de Janeiro & Sao Paulo), Denmark, Estonia, Finland, New Zealand, Poland, Sweden, the United States and the UK.
Skype Lite is mainly useful for Skype IM and cheap international calling. You can make calls to Skype numbers anywhere in the world for the cost of a local call. You can also call international mobile and fixed line numbers using Skype Out which will almost always be cheaper than your mobile operator's rates. Skype IM is fully supported and uses only data.
I like Skype and use it on my PC quite a bit. It's far and a way the biggest free VOIP network and the best part is that it just "works". It's ultra reliable, there's nothing to setup and call quality is generally excellent.
Skype's been promising a true VOIP enabled mobile client forever. They released one for Windows Mobile years ago but we are still waiting for a S60 version. Tuesday at MWC, Skype and Nokia finally announced a S60 Skype client that supports free Skype to Skype calling on WiFi or 3G. The only catch is that it will only available as part of a firmware upgrade. Given that Nokia hardly ever issues firmware updates for the US versions of their phones I doubt I'll ever see Skype on my N95-3. I don't understand why a Skype client can't be a simple .sisx download. Gizmo, Fring and TruTap all delivered stand alone S60 clients two years ago, why can't the much larger, Ebay owned Skype? Source: Oh! Mobile Directory