First, if you know nothing about *, download the book Asterisk: the future of telephony for free (creative common license) from asteriskdocs.org. It will edumacate you 
IMHO, grandstream are not good for a business environment unless you are really pinching your pennies (GXP2000 is about $80/unit as i recall). The phones (esp. budgetone but still somewhat the GXP) feel lightly built. Also, their remote provisioning system is sort of odd and poorly documented (by GS, it’s decently documented by the Wiki, but still requires their converter tool to turn text files into config files the phone understands).
I second the recommendations for Aastra 480i/9133i and Snom 360. I have used all of these phones and they are all a joy to work with. They feel good and solid in your hand, real business quality product. Both aastra and snom 360 have good usable speakerphone. All three support 802.3af power over ethernet.
Also for easy administration, both use a useful config file format- Aastra is just a plain text file with params ie
sip line1 auth name: AJohnson
so it is exceedingly easy to generate them in batches from a script and a database. Snom has a similar format which is more versatile and as I recall somewhat scriptable, haven’t played with it though.
For secure provisioning, i know the aastra lets you symmetrically encrypt the config file with a tool they provide for win32/linux/solaris or you can have the phone pull the cfg file over https. I think snom has the same thing.
Snoms have the interesting ability that they can pull ringers as needed. Place an http link to a correctly formatted wav file in the sip header and the phone will download the ringer and use it for that one call (Asterisk can do this). Makes setting distinctive rings from a server VERY easy. AAstra phones are currently incapable of downloading custom ring tones, you are stuck with 5 or 6 ‘warbles’. Both allow you to choose a warble by sip header.
Snoms are generally very versatile… the snom 360 ‘advanced options’ page has like 50 things that can be turned on and off (dial when you pick up? disable speakerphone? no incoming intercom? that type of customizable tweaks) which can be quite useful.
Both AAstra 480 (NOT 9133) and the Snom 360 have XML microbrowsers built in so you can use the large display to access various services. This may be nothing at all, and they are not required, but they are easy to write.
I don’t recommend polycom as much, although they are very well built, have awesome speakerphones and support xml. The reason is, polycom is a bunch of annoying pricks when it comes to firmware. They refuse to make the latest firmware or documentation available on their website to anybody that isnt a ‘polycom certified reseller’ or whatever, and finding one to help you isn’t always easy. There are a few 3rd party sites that redistribute the firmware but I hate relying on such things for a business setup. Polycom’s MO is that they want you to buy a vertically integrated system from a reseller, who will then be the only one you deal with.
In contrast, AAstra and Snom are both happy to deal with users and generally try to make themselves useful for people that want answers or files w/out jumping through hoops.
Snom provides firmware off their website (you can even upgrade by punching the snom.com download link straight into the phone) and has an extensive Wiki of their own. Also, i believe one of their guys posts on the voip-info wiki (user: snomy is from Snom IIRC). Firmware releases from Snom often address issues brought up on the Wiki pages.
I have heard AAstra is very responsive via email as well. They don’t release as often but it is usually worth the wait. Many ‘major’ features that have been requested on the Wiki have been added (BLF, XML, distinctive ring by sip header, etc). They provide very extensive and useful documentation in PDF format, free to download. Firmware is also freely available.
Hope that helped! 
