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So Windows Vista finally allows to enable ClearType font smoothing for Remote Desktop / Terminal Services sessions.
If you try to connect to a machine running Windows Vista using rdesktop, you won’t get smoothed font typing since at the time of this writing rdesktop does not officially offer an option to control this feature. However, here is a workaround:

rdesktop allows to specify the RDP5 experience via the -x experience switch.
One can either define one of three default experiences (modem, broadband, lan) or one can specify a raw hex value that is send to the server.

NOTE: You can skip over this rather technical part, if you’re not interested in the details. You’ll find the workaround below.

This hex value is actually a combination of defined bit flags. After some tinkering I found that the hex value 0×80 will enable font smoothing for the connection.
The file constants.h of the rdesktop sources contains these flags:

#define RDP5_DISABLE_NOTHING	0x00
#define RDP5_NO_WALLPAPER	0x01
#define RDP5_NO_FULLWINDOWDRAG	0x02
#define RDP5_NO_MENUANIMATIONS	0x04
#define RDP5_NO_THEMING		0x08
#define RDP5_NO_CURSOR_SHADOW	0x20
#define RDP5_NO_CURSORSETTINGS	0x40	/* disables cursor blinking */

So, naturally an additional flag constant can be defined like this:

#define RDP5_ENABLE_FONT_SMOOTHING 0x80

The file rdesktop.c would have to be extended preferably with an additional argument that controls the font smoothing.
If you want to use font smoothing with rdesktop now you have to combine the flags (bitwise OR, addition will do too) and specify the result via the -x switch.

Here is the workaround for the three defaults mentioned above:

rdesktop -x 0×8F mywinserver   # equals the modem default + font smoothing
rdesktop -x 0×81 mywinserver   # equals the broadband default + font smoothing
rdesktop -x 0×80 mywinserver   # equals the LAN default + font smoothing