Your post title is somewhat misleading and should read as T-Mobile does not "support" audio streaming, since you've concluded that T-Mo may be blocking it.
Perhaps the ability to stream audio is more to do with the availability of the codec rather than the OS, or perhaps Google was not able to secure the necessary licencing for the needed codec?
I'm an N800 user as well, and I think people need to understand the intent of the device that they have bought. For the longest time, NIT users have been whinging about the unavailability of a PIM app, the absence of an Office-compatible reader/writer application, etc. Yet many fail to realise that the device is sold for the purpose of being an Internet Tablet -- it's great for surfing the web, emails, streaming content, VoIP calling, instant messaging, etc, and the ability of the device to take photographs with the built-in VGA camera, and play back videos and music is a BONUS.
Looking at the T-Mo G1 site at
http://www.t-mobileg1.com/, it shows that the phone does the following:
Web browsing (working well)
Sending and receiving emails over POP3/IMAP3 (working)
'Push' Gmail (working)
Contacts & Calendar with Gmail/GCal push sync (working, but I wish they'd found a way to integrate group SMS/MMS/Email functionality)
Picture Viewer (working - i love the autorotate and wish they could get this working with apps like the web browser, contacts, calendar, etc)
Making and receiving calls on GSM/UMTS networks (working, but reception quality could be better)
Sending and receiving SMS/MMS (SMS working, but I don't have the MMS settings for my mobile service provider. Also, I wish they'd allow for longer SMS messages before converting it to MMS, and sending to groups)
Web searching via Google (working)
Multiple 'Today' screens with Application shortcuts, Widgets, etc (working)
Android Market (working, and growing quickly too)
Google Maps (working, but would be useful if they allowed to set up my own POIs and sync that with Google Maps on my desktop)
Instant Messaging (working over GTalk, but Windows Live Messenger and Yahoo Messenger seem limited to work only over T-Mo's data network or WiFi)
YouTube (working)
MP3 music player with contextual menus to Amazon, YouTube and Google searches
The way I see it, anything that I can get off Android Market is a BONUS, e.g. ability to read barcodes, stream music (Imeen), watch MP4 videos, etc is a BONUS, something the G1 is capable of doing, but was not designed to do.
An example would be a family MPV. You buy one because you can fit more than 5 people in it and it's comfortable for long trips, but you know it would be silly to complain about the poor handling and acceleration on track, when comparing to a sedan or a saloon.