[britta]

Do you mean the JailbreakMe web-based exploits? JailbreakMe is not available for iOS 4.2.1. It's available for 3.1.2-4.0.1 and 4.3-4.3.3 - see http://en.wikipedia.org/wiki/JailbreakMe#Compatible_iOS_vers... for details.

[Xuzz]

Yeah, but there's nothing stopping someone from porting JailbreakMe to 4.2.1 — it already works on 4.2.8 for the Verizon iPhone 4, and the other versions are similar enough. The hardest part would be the CPU (armv6 vs armv7), but it's clearly possible.