6to4 on Sprint Wireless Broadband

A few months ago, is appears that Sprint started dropping 6to4
encapsulated packets. Egress is fine. Ingress silently drops. Anyone
see anything similar? Or am I the only guy crazy enough to be doing
this sort of thing in the first place?

6to4 is a method for tunnelling v6 over v4 without statically-configuring endpoints. The encapsulation used is protocol-41, i.e. the same as "tunnel mode ipv6ip" in IOS or "interfaces ip-a/b/c" in JUNOS. It seems unlikely that Spring has decided to block IPv4 packets with protocol 41 payload.

What seems far more likely is that the relay routers used for return packets from the hosts you are trying to talk to are hitting a broken 6to4 relay router, and hence the return traffic is being (from your perspective) silently discarded.