Here's a look at what Apple, Nokia, and RIM's proposed nano-SIMs look like. The Verge got a hold of the proposals from each company and notes that new card must support eight electrical contacts.
Apple maintains the old configuration; in other words, with an adapter, you could theoretically use their nano-SIM in a micro-SIM or mini-SIM phone sold today. Nokia and RIM take a very different approach. The 4FF proposals from Apple's competitors look more like microSD cards than present-day SIMs, which would presumably make it difficult or impossible to use an adapter to get them working with older phones.
Nokia has argued that its proposal, unlike Apple's, would not require a tray. The company also argues that Apple's design could get stuck in a micro-SIM slot violating the following requirement for the standard:
The design of the fourth UICC form factor shall prevent the 4FF from becoming jammed in a Mini-UICC reader. An example is that if the 4FF is turned 90 degrees and it fits perfectly into the Mini-UICC reader (4FF length = Mini-UICC width).
The European Telecommunications Standards Institute will make a decision on the standard at the Smart Card Platform Plenary meeting later this week. Apple has committed to grant royalty-free licenses if its nano-SIM standard is adopted.