It is possible to install Exchange Server 2007 with full OWA and ActiveSync capability on a single server (this was not possible with previous versions without an additional front end sever). However, whatch out for these pitfalls:-
1) Root web site will become HTTPS only after installing Client.
2) Without the service pack 1 (currently in Beta) the Information Store service will fail to start because it starts too early and cannot find the domain controller. To fix this edit the registry key HKLM\System\CurrentControlSet\Services\MSExchangeIS\DependsOnService and ADD (do not delete the existing entries) on a new line (i.e. at the bottom) "MSExchangeADTopology" (no quotes). This will ensure th IS does not start before the Active Directory, as the domain controller is the same machine it may take some time to start. I recommend REMOVING this additional line before applying any service packs, and adding it back in again only if the problem is not fixed in successive service packs (at the time of writing, none were available).