Not sure what VLAN you were planning to make the backbone. Are the 3550's housed at one of the listed sites along the left? Are they near the servers?
One thing that occurs to me is that if the 3550's are in the same locale as the servers, then you might want to look at hanging the servers right off the 3550's.
Also, I'm not sure I would mix 2900 and 3500 series switches. They're in the same ballpark as far as price, and the 3500 has alot of advantages over the 2900s. Either go with all 2900s or all 3500s. It also seems wrong to put the slower 2900s at the server location, as this is where most of your traffic is going. If you connected the servers directly to the 3550s at the outset, you could start with 100TX and move straight into 1000T when you need a bigger pipe.
The final consideration regards spares. Try to stay with as few different device configurations as possible. This will make having a spare switch available for failures much easier.
It seems you could do this config with a mix of two or three devices, as follows:
-3550's in the core, 2950's everywhere else
-3550's in the core, 3524XLs everywhere else
-3550's in the core, mix of 3524XLs and 3548XLs everywhere else
-3550's in the core, 3550's in the server section, either 2950 or 3524/3548s everywhere else.
You're in good shape, just some product selections need to be looked at to make sure they really are the right devices.
