Some people in #samsung-fascinate were complaining about slow 3G download speeds. Here is a simple fix:
Create a file on your PC called sysctl.conf, add the following information and then adb push sysctl.conf /system/etc/sysctl.conf.
net.ipv4.tcp_ecn=0 net.ipv4.route.flush = 1 net.ipv4.tcp_rfc1337 = 1 net.ipv4.ip_no_pmtu_disc = 0 net.ipv4.tcp_sack = 1 net.ipv4.tcp_fack = 1 net.ipv4.tcp_window_scaling = 1 net.ipv4.tcp_timestamps = 1 net.ipv4.tcp_rmem = 4096 39000 187000 net.ipv4.tcp_wmem = 4096 39000 187000 net.ipv4.tcp_mem = 187000 187000 187000 net.ipv4.tcp_no_metrics_save = 1 net.ipv4.tcp_moderate_rcvbuf = 1
I'm continuously tweaking these settings, they are desktop settings, but I was able to go from 900kbits/sec on 3G to 1800kbits/sec. Your mileage may vary of course. I live in a rural area so 3G usage is probably lower than in say Chicago or New York.
*Not responsible if your phone quits working, catches on fire or threatens to kill a foreign head of state. Also not responsible if your download speed doesn't increase. And I'm not going to support this or hold your hands. If you don't know anything about Linux, Android, ADB or editing system files, don't bother me. =)