I don't have my Droid yet (getting it tomorrow), so I'm just speculating here, but let's say everyone is right. Perhaps Verizon does split up SMS messages to non-VZW customers. I know they split them up for VZW customers, and I'm pretty sure this is a carrier thing, not a phone thing. Perhaps, also, Verizon converts MMS messages to a single SMS (160 chars. max) for non-VZW customers. The Droid apparently has a feature that converts 160+ character SMS messages into MMS messages (this is definitely a Droid thing, not a carrier thing). However, you can't send MMS messages to non-VZW phones, so Verizon converts it back to a SMS. Perhaps it is the conversion process from MMS back to SMS where you are actually getting cut off. If this is the case, then it seems that this could be overcome by turning off the option to convert long SMS messages to MMS... if you can turn it off.
Just my $0.02.