As mentioned above, it became required by law to include TPMS in all US vehicles later in 2007, so almost all 2008 model vehicles got it but not all 2007 models did. Also, I believe the TPMS used by Mercedes-Benz in 2008+ E-Class models is the "low-level" version, which uses ABS to detect the speed of the wheels and if one wheel speed changes by a set variance, it warns you that a tire is low on pressure but doesn't tell you the actual reading. The "high-level" version actually has four separate sensors each reading and displaying individual tire pressures.
Having said that, Steve@MBenzNL can install the latest version of the Mercedes "high-level" TPMS in ANY car and does a great job at it (probably even a little better than OEM). Feel free to shoot him an e-mail, I've seen his work, and it is absolutely excellent