I recently upgraded from 4.0.1.something to 4.1.0.309 on my T-Mobile BIS Blackberry (Edit: model 7290). I couldn't find any real descriptions of what had changed prior to performing the upgrade, so I went ahead with it to find out. For the benefit of others considering it, here's my take on it:

The good

• Phone volume improved somewhat (although not as much as I would have liked). This is a subjective assessment; didn't check with test equip.
• Web browser shows total amount to be downloaded on progress bar in kb
• Web browser can finally zoom in/out on images like the email attachment viewer can.
• Web browser can display larger images - things which used to say "too big too display" now work for me
• Per-caller ringtones/profiles finally available, like every other cell phone has had for 5 years
• I no longer get random tunnels errors half the time when using 3rd party TCP applications
• MMS support is now included (I haven't used this at all yet)

The bad

• The "compose" icon disappears from the main screen, so compose is no longer a one-key command (but you can get an add-on program to bring back something very simliar)
• Half the options are now in a submenu under "Advanced Options".

The ugly

• Font sizes changed, and look worse now in my opinion. The new Millbank 7 is somwhere in between what used to be 6 and what used to be 7. 7 used to be just right for me, but the new 7 is too small and the new 8 is too big. If you restore Options from an old backup, you can get the old size back, but you can't pick it from the UI anymore.

The just different

• By default, you now get a messages icon PER ACCOUNT on the home screen. I found this useless and hid the icons, but maybe you'd want it.
• Under network, you can turn off data entirely if you want (I guess in case you're roaming and don't want to pay data charges for the random housekeeping traffic)
• During power off, it only displays "press any key to abort" for about half as long, and displays the provider logo for the other half of the time.

The same ol'

• Still no way to remove the useless default bookmarks
• No way to obtain any alternative non-default vibration patterns, so in vibrate mode, the per-caller profiles do you no good whatsoever
• Web browser still can't do anything with PDFs, even though the email attachment viewer can

Anything I missed?

Yes. What model BB are you using?

7290. I knew I'd forget something important.

I share Master Pizzle's curiosity. One additional note: your upgrade process bypassed OS .284 which already instituted many of the changes to which you refer and which is now the standard OS on new T-Mobile and Rogers 8700s.
EDIT: I can't speak to the issue of OS changes on the 7290.

yup, good assesment. i just upgraded my nexel 7520 to 4.1. i was shocked they ditched the compose icon. now my spellchechker wont work.

I've found a few more things since I originally posted.

The title screen now ALWAYS says "T-Mobile"; it used to display the name of the network one was on when it was someone else's, but they have changed it for better advertising and less useful information. This is probably a per-carrier setting so it may not be the same for the others. T-Mobile's also says "Searching..." when it should say "Emergency Service Only"; I think this is intentionally deceptive, but the old version did that as well.

It is now possible to save voice mail notifications to the message list, but they still don't include the phone number that left the message or any other useful information, so there is little to be gained by doing so.

The main screen icons changed how they're disabled; when "show all" is turned on, the normally-off icons are now slightly faded instead of Xed out. I find it harder to read. The same is true for the bluetooth icon when it's turned off. The "profiles" icon has variants which show if one of the default-named profiles is selected.