I am about 2 miles from Google HQ today (Mountain View, CA) and I ran speed tests on both APNs I use.
Name: ATT HSPA+ (this shouldn't matter)
APN: phone
MMSC: mmsc.mobile.att.net
MMS Proxy: proxy.mobile.att.net
APN type: default, supl, mms, hipri
Speeds: 108ms, 5.8mb, 1.1mb
Name: ATT
APN: wap.cingular
MMSC:
http://mmsc.cingular.com/
MMS Proxy: wireless.cingular.com
APN Type: default, supl, mms
Speed: 140ms, 3.3mb, 1.1mb
I'm on WiFi a majority of my day so I tend to keep it on the wap.cingular APN because I like my emojis when texting, handcent functionality and ease of sending pics over text. When speed matters I switch back to the phone APN.