The version in cydia has been broken for quite some time. Newer versions are available out on the google code site, but they're pretty rough around the edges.
Mobile terminal works, just not the version from cydia. the one here works, if you place it on your phone and open it using iFile or the like, you can get a working version of mobile terminal.
Yes you still need to change your password, if you've installed openssh.
Prompt and iSSH have some features that Mobile Terminal doesn't have (even the latest version) - various gestures, extra keyboard buttons, etc.
You can also view top locally on your device if you install Mobile Terminal (you can download a working version here) - you can install the .deb file using iFile for example.
I don't know what wifid using a lot of CPU means, unfortunately. Here's a list of some iOS daemons, but I guess it doesn't list all of them.
> I also miss having my contacts synced with my google contacts and my facebook friends (their pictures).
This is very possible. Don't set up your Gmail account with the default Gmail profile, but use it as an Exchange account as detailed here.
As for Facebook pictures, download and open the Facebook app, go to Friends>[Arrow thingy]>Sync Contacts then turn "Syncing" on and "Replace photos" on. Leave "Replace photos" off if you want to keep your gmail pictures, but I prefer the oft updated Facebook pictures.
> Is there a terminal application or a file browsing interface?
Only if you jailbreak (which I assume you'll do). MobileTerminal is the main terminal option, however the one available through Cydia won't work on iOS 4. Download the .deb file from here and copy into /var/root/Media/Cydia/AutoInstall (you may have to create it). Soft reboot your phone twice and it should show up as a normal app.
>I want to change the "new mail" sound. (Best way to do that?)
You'll just need to figure out which file it is. (Look in /System/Library/Audio/UISounds/, it's probably descriptively named.) Here are instructions for actually making the changes, or if you're savvy already you can use SSH and SCP.
Edit: > what's the best way to jailbreak it?
If you're on Windows, redsn0w. If you're on Mac, PwnageTool. Get to both from here
Edit again: Since you need OpenSSH to make the changes you want, be sure to change your password from its default (alpine)! Use MobileTerminal from here as the Cydia version is typically out of date.
So, that error message is confusing. Cydia automatically runs the dpkg command by itself, and the freezing indicates that something has gone wrong after running the command. You can try running the dpkg command manually, but it might not be helpful either. If you want to run the command, download the latest MobileTerminal version and install it with iFile or similar.
Did you do anything weird in Cydia before this happened? What do you mean by an interruption in the stream? You might just need to delete a cache file or something.
Yes, there is a google code repository with the most recent version. Although you need a third party utility, such as ifile, to manually install the Debian file.
Edit: Here's a link: http://code.google.com/p/mobileterminal/