There are regional and phone network restrictions for Skype. They may well be different for Nokia phones than Android phones, and your friend may not be on the same phone network as you.
There are (I think) four official Skype apps in the Market, they all have different restrictions:
Skype mobile on Verizon - requires Android 1.6+, Verizon USA customers only
Skype for au - requires Android 1.6+, au Japan customers only
Skype on Three- Three UK customers only
Skype - requires Android 2.1+, works on all phone networks (outside of the US) where your phone network allows it.
The above options for finding Skype in the Android Market only work if your country has its own Android Market, if your country currently uses the "Rest of the World" Market, then you won't be able to find Skype. According to Skype's FAQ, if this is the case then you should go to GetJar.com and download Skype from there.
Assuming you're not in a country/network that's blocking Skype then your problem may be the Android OS version on your phone, I think most Xperia x10's are still running Android 1.6 and the normal Skype app requires that you be running Android 2.1 or newer.
I think most countries have an Eclair (Android 2.1) upgrade for X10's, check what Android version your phone is running, and then find out if an upgrade is available.