The only other thing that would still be on the switch after a 'wr erase' would be the vlan database file, since on the 3500xl platform you had to do all the vlan configs from 'vlan database' mode. However, I don't see how that would interfere with layer3 connectivity to your switches vlan1 SVI.
Where are you trying to ping the vlan1 SVI from? The same switch from a PC connected to a vlan1 interface, or have you trunked this 3500xl to another switch and are trying to ping it from there?
Where ever you are trying to ping the vlan1 SVI from, I would issue the 'sh int ' and check to see if you have any CRC, FCS, or other errors. Do your logs on the 3500xl have anything in them?
