Hi Marcus
The SR520-T1 is orderable now with distis like Ingram and DnH
In terms of performance it can support a full T1 data throughput and upto 10 IP Sec or SSL VPN tunnels.
The 1800 platform comes with a lot of advanced routing/security functionality and TAC support whereas the SR520-T1 is mainly for the small business customers/partners who can utilize the embedded simple to use configuration utility for setting up the T1 data interface and CCA 2.1 support for the security features to provide a reliable T1+security solution.
Cheers
Jay