handsets with 512MB or less of ROM (internal memory) will not be getting updated which means no Ice Cream Sandwich for the HTC ChaCha, HTC Salsa, HTC Wildfire S, HTC Explorer and others. Looking at tablets, the HTC Flyer, the HTC EVO View 4G and the HTC JetStream will all stick with Honeycomb.
It's interesting that this is a situation similar to what we saw last year with the HTC Desire. The handset didn't get bumped up to Gingerbread because it only had 512 megabytes of internal memory. HTC's statement that 512MB of ROM was the cutoff point for deciding which models which get Android 4.0 seems to confirm that this is a cut off point and devices like that might not be able to smoothly run Ice Cream Sandwich.
source: HTC via Phandroid