There is an issue with installing SP2 for Exchange 2007 on SBS 2008, Microsoft has released a tool to get passed the errors - link. You need to download and extract Exchange 2007 SP2 - link, run the install tool. It will ask for the directory of SP2, and then you install SP2 normally.

Here are more details of the issue on MSExchange.org

We recently implemented a new SBS 2008 server for one of our clients.  A user was going to be working from home and wanted to utilize thier corporate office desktop to get some tasks done.  They were able to authenticate to the Remote Web Workplace webpage successfully,  but were unable to connect to their Windows 7 desktop computer.  Every time they attempted to do so, their account was rejected with an error message: “The logon attempt failed”

I discovered the problem was due to an incorrect setting within IIS.

Here’s what I did to correct the problem:
I went to IIS Manager → Sites → SBS Web Applications → Rpc → Authentication.  There I found only “Basic Authentication” was enabled.

Enabled “Windows Authentication” then ran an IIS reset. When IIS came back online, I was able to connect via TS Gateway to the server and at least one workstation. I connected and disconnected multiple times and it had no problems. The end user verified they were also able to authenticate without error, and gained remote control of their Windows 7 computer.

You run into issues installing Exchange 2007 SP2 on SBS 2008 until you add the following key and DWORD to the registry.

To create the E12SP2READY registry value, follow these steps:

1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\Software\Microsoft\SmallBusinessServer\Exchange

   Note If the Exchange subkey does not exist, you must create it. To do this, follow these steps:

   1. Right-click SmallBusinessServer, point to New on the Edit menu, and then click Key.
   2. Type Exchange and then press ENTER.
3. After you select the Exchange subkey that is specified in step 2, point to New on the Edit menu, and then click DWORD Value.
4. Type E12SP2READY, and then press ENTER.
5. Right-click E12SP2READY, and then click Modify.
6. In the Value data box, type 1, and then click OK.
7. On the File menu, click Exit to exit Registry Editor.

Full article at Microsoft - http://support.microsoft.com/kb/973862

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There are additional steps to get Exchange 2007 SP2 installed on SBS 2008, below are the steps to complete from Microsoft Support

http://support.microsoft.com/default.aspx?scid=kb;EN-US;973862