Disclaimer: The Android system does not officially support writing to the SIM card. That means that manufacturers probably don't test this feature, and as a result it may or may not work, depending on the phone model. And as Contact2Sim ultimately relies on the system to perform its actions, there is no way to guarantee it will work either.

Works only on GSM phones

Contact2Sim needs a SIM card to read from and write to. Usually GSM networks use SIM cards. CDMA networks don't. So a phone running on Verizon or Sprint cannot be used.
And of course the radio must be started (not in Airplane Mode) !

Android 1.x

Contact2Sim works fine on most phones running Android 1.5 ("Cupcake") or 1.6 ("Donut").

Android 2.x

Android 2 ("Eclair", "Froyo") brings a specific support for USIM cards. These cards, used with almost all 3G phones, are an evolution of the plain old SIM card. Unfortunately, only the read part has been taken care of in Android. The write part is broken. The regular SIM cards work just as before, but Contact2Sim cannot use USIM cards, except for the "Airplane Mode" trick (see below).

July 11th, 2010 I'm currently working on a patch for Android to fix this. If it's accepted by the Android dev team, full functionality will be restored in a later version of Android. Check for updates here.

July 24th, 2010 Google accepted my patch and merged it into the current version of the Android system. Many thanks to the Android dev team at Google for their support! That means that from now on, the phone manufacturers can produce system updates including this fix. But whether and when they do it is up to them...

August 30th, 2010 CyanogenMod, the most popular custom Android build, now includes my fix. I tested CyanogenMod 6.0 with Contact2Sim on Nexus one, and it worked like a charm! On several phone models, there is now no need to wait for the manufacturers to release an update!

The Airplane Mode trick

On some Android 2.x phones, switching the radio off and on again (Airplane Mode on and off) makes the phone forget it has an USIM card and think it has a regular SIM instead. Contact2Sim works then normally. This works well on the Nexus One.

SIM card limits

The SIM card is limited both in number of entries (typically 250) and length of the names, which may be truncated. Too long numbers may not be copied at all. And due to a limit in Android, SIM cards having a storage capacity greater than 250 numbers may still be limited to 250.

Automatic name truncation may fail on some phones. In that case, contacts with too long names are not copied at all to the SIM. It is then necessary to set the limit manually in the settings.

Non-latin characters

Contact2Sim reads any kind of character from the SIM card (Chinese, Russian, etc...). But when writing back to the SIM, non-latin characters are lost. Again, the issue lies in Android itself and there is no solution for now.

For further information, use the contact e-mail here.