Apple ships the new Generation of iPad with default SIM cards called 'Apple SIM', giving users the choice which provider and subscription they want to enroll with.
For now only in the Unites States and UK, iPads will have this new 'feature'.

While Apple did not mention the special SIM cards during the presentation of the new 4G enabled iPads on Thursday, however the US Apple product site mentions "One sim, many possibilities." The manufacturer states that the SIM card is already preinstalled within the Air, iPad 2 and iPad 3 mini series, along with WiFi and mobile internet support.

The iPad user will have the choice, which cell provider and subscription he prefers. According to Apple, any subscription can be changed at will and while traveling, people will be able to switch to a local cell provider subscription for the duration of the travel and thus avoid expensive roaming costs.

Further information on the SIM cards are not yet available and whether Apple will release this SIM card model in other countries and for the iPhone is not yet known.
Already in 2010 reports surfaced about Apple working with SIM card maker Gemalto on embedded SIM cards.
 

According to the Financial Times, cell providers back then cautioned about embedding this type of system. Objections on their behalf are understandable as it would become almost too easy for subscribers to switch operator.
For now only AT&T and Sprint for the Unites States and EE for the United Kingdom are the official Partners of Apple.
In all other countries, people will continue to rely on existing cards as the SIM card tray remains available on iPads.

According to The Verge, the SIM card is a dying breed. Apple might take this a step further and ship SIM card free devices with embedded SIM chips.

Sources: DataNews, tweakers, MacUser, GIGAOM